Abstract: The low utilization rate of agricultural machinery can be caused by the fragmented and scattered farmland plots with the diverse planting patterns in southern China. In this study, a single-seed precision seed-metering device was developed with a pneumatic roller for multiple crops using theoretical analysis and bench tests. Maize and rapeseed were selected as the experimental subjects due to their significant morphological differences. A systematic evaluation was also performed on the device's adaptability. The overall structure and working performances of the device were highlighted to combine the positive and negative air pressure for the precise seed metering. The structural parameters of the seed-metering hole wheel were determined according to the normal distribution in the triaxial dimensions of the seeds. The seed-metering wheel was equipped with the spoon-shaped holes (with a diameter of 130 mm) that were tailored to each crop. Specifically, the hole dimensions were as follows: The length of 2.5 mm, the width of 2.4 mm, the depth of 1.8 mm, the suction hole diameter of 1.3 mm, the spoon head radius of 1.7 mm, the angle between the spoon tail and the horizontal direction of the suction hole was 45° for rapeseed, while the length of 15 mm, the width of 14 mm, the depth of 8.5 mm, the suction hole diameter of 4.3 mm, the spoon head radius of 7 mm, and the angle between the spoon tail and the horizontal direction of the suction hole was 30° for maize. The mechanical analysis was conducted on the seed-filling, seed-carrying, and seed-throwing. The relationships between these processes and key parameters were established, such as the air pressure and the structure of holes. Furthermore, the performance was optimized for the forces acting on seeding. Single-factor experiments identified the optimal working ranges for the suction negative pressure and rotational speed. The suction negative pressure of maize was 3-5 kPa, and the rotation speed was12-20 r/min. In this range, the qualified index of maize seeding could reach 94.88%, and the minimum values of multiple index and leakage index were 2.70% and 0.99%, respectively. The negative pressure of rapeseed suction was 1.1-1.9 kPa, and the rotation speed was 15-35 r/min. In this range, the qualified index of rapeseed seeding could reach 96.4%, and the minimum values of multiple index and leakage index were 1.11% and 1.02%, respectively. The results demonstrated that the performance of the device was achieved with high precision under diverse operating conditions. A two-factor five-level orthogonal rotation experiment was also conducted to further refine the operational parameters. Variance analysis was then used to verify the optimization. Three-dimensional response surfaces were generated using Design-Expert software. There were relationships between working negative pressure, rotational speed (experimental factors), and the qualified, multiple, and leakage index (evaluation metrics). In the corn qualification index above 94%, the stable working range was negative pressure 3.3-3.7 kPa and rotational speed 13-16 r/min. In the rapeseed qualification index above 94%, the stable working range was negative pressure 1.2-1.8 kPa and rotational speed 18-30 r/min. In maize, the qualified index was 95.63%, the multiple index was 3.52%, and the leakage index was 0.85% at a rotational speed of 13 r/min (forward speed ≈ 3.6 km/h) and suction negative pressure of 3.3 kPa. In rapeseed, the qualified index was 95.60%, the multiple index was 2.92%, and the leakage index was 1.48% at a rotational speed of 23 r/min (forward speed ≈ 3.4 km/h) and suction negative pressure of 1.50 kPa. A modular seed-metering device was developed for the single-seed precision seeding compatible with both large and small seeds. The finding can provide a strong reference to improve the quality of mechanized sowing. Technical support can also be offered for the multi-purpose use of a single machine under diverse planting patterns in the clay-heavy soils of southern China.