Abstract: A scallion is one of the most favorite vegetables in the world. However, manual and semi-mechanized operations cannot fully meet the large-scale harvesting of the scallion industry in recent years. Some challenges also remained in mechanized scallion harvesting, such as soil clogging and crop damage. This study aims to present the design, key component optimization, and field validation of the 4 LQ-1 side traction scallion combine harvester. Five subsystems were integrated in the harvester: a depth-limited device, a combined excavation device, a conveying and soil-cleaning device, a lateral laying device, and a transmission system. The sequential operations were then realized, including the ridge-side soil separation, excavation and lifting, clamping and conveying, rotary soil cleaning, and lateral laying in a single pass. Technical parameters included an overall dimension of 3 600×2 100×1 400 mm (length×width×height), a matched power of no less than 88.2 kW, a working width of 0.25 m, a total weight of 436 kg, a three-point hitch connection, and a working speed range of 0.3-0.4 m/s. One key part of the harvester, the combined excavation device consisted of a soil separation and an excavation-lifting mechanism: The soil separation mechanism was used the rotating spiral blades (with the maximum diameter of 250 mm and a shaft diameter of 75 mm) to cut, crush, and push soil from both sides of the scallion ridge, thus reducing the soil volume that handled by the excavation shovel, in order to prevent the soil clogging; The excavation-lifting mechanism was adopted a trapezoidal digging shovel (275 mm in width and 120 mm in length) to minimize the excavation resistance, paired with lifting bars (250 mm in length and 75 mm in spacing) that loosen and lift the soil-scallion mixture, thus allowing crushed soil to fall through the bar gaps during lifting. The conveying and soil-cleaning device comprised a clamping and conveying mechanism, as well as a rotary soil-cleaning mechanism: The clamping mechanism was used the flexible foam rubber belts (100 mm in width) to gently grip the scallion at the junction of leaves and the white part (scallion bulb), with a feeding inlet width of 300 mm (matching the average leaf spread of mature scallions) and a depth of 600 mm, in order to obtain the stable clamping without damaging the crop; The rotary soil-cleaning mechanism was employed the rubber rods rotating at 20 r/s to strike and remove adhering soil from scallion roots, thus balancing the cleaning efficiency and crop protection (in order to avoid the damage caused by excessive rotation speed). The transmission system was used to draw the power from a tractor: the tractor’s power take-off shaft was connected to the harvester’s main drive shaft via a universal joint and an HD input commutator, thus powering the soil separation mechanism (through an HD output commutator and chain drive) and the rotary soil-cleaning mechanism (through chain drive). The tractor’s hydraulic system was used to drive the clamping belt’s hydraulic motor and lifting cylinders. The lateral laying device was powered by the harvester’s own electrical system. A three-factor and three-level orthogonal experiment was conducted to optimize the performance. The variables were taken as the forward speed, soil separation mechanism rotation speed, and clamping belt inclination angle. While the evaluation indicators were taken as the leakage rate (unharvested scallions), damage rate (structurally damaged scallions), and excavation rate (successfully harvested scallions). The experiment was carried out in a sandy loam field in Jiaozhou, Shandong Province, China, in May 2024. The soil moisture content was 8.3%, soil compaction was 0.6 MPa, and scallions were planted with 269 mm ridge height, 801 mm ridge spacing, and 62 mm plant spacing. Mathematical models between factors and indicators were established using Design-Expert software. The optimal combination of the parameters was determined: a forward speed of 300 mm/s, a soil separation mechanism rotation speed of 200 r/min, and a clamping belt inclination angle of 30°. Field validation tests (three replicates, each covering 100 m of ridge) showed that the leakage rate was 0.8%, the damage rate was 2.85%, and the excavation rate was 98.7% under the optimal parameters. The absolute error between the measured and model predictions was within 5%, fully meeting the technical requirements for the scallion harvesting equipment (leakage rate ≤5%, damage rate ≤5%, and excavation rate ≥90%). This finding can provide the theoretical basis and technical support to the structural and performance enhancement of the side traction scallion harvesters, in order to effectively reduce the harvesting costs for the high operation quality of the ridge-planted scallions.