Abstract: Efficient and accurate seedling is often required to accelerate agricultural modernization. Among them, solar greenhouse seedling cultivation is the initial stage of seedling cultivation, particularly for high yield and quality. However, the conventional seedling cultivation in solar greenhouses is limited during harvesting and releasing the seedling trays. It is difficult to fully meet the demands of modern agricultural production due to its manual labor and simple equipment for unidirectional movement. Improper operation is also prone to damaging the seedlings, leading to low-quality seedlings. Therefore, it is often required for efficient, precise, and large-scale seedling cultivation. In this study, an automatic retraction and extension bidirectional conveying device was designed for the placement, transportation, handling, and storage of seedling trays. The seedling trays were precisely grasped to achieve the automatic placement and bidirectional transportation of the seedling trays. A systematic analysis was made to explore the key technologies and motion parameters of the bidirectional retraction and extension conveying device for seedling trays. A series of single-factor experiments was carried out. The main influencing factors on the transportation performance were then determined as the speed of the conveyor belt, the power of the motor, and the device transportation speed. The test indicators were also taken as the dropping rate of seedling trays and the seedling tray transportation speed. The photoelectric sensor was utilized to receive the light reflected by the seedling trays on the conveyor belt during the experiment. The speed of the conveyor belt was determined according to the light intensity and time intervals of the reflection. The speed and power of the motor were regulated by the power supply frequency through a frequency converter, thereby adjusting the speed of the transportation device. Correspondingly, the value ranges of each factor were determined to be 0.28-0.36 m/s, 320-380 W, and 4.5-5.5 m/min, respectively. A Box-Behnken experiment was also conducted for the regression fitting on the variance of its values. The results showed that there were significant regression models of the seedling tray drop rate and seedling tray transportation speed, indicating a better fit with the experimental values. While there was no significant difference in the unfitted terms. The corrected determination coefficients R² of the seedling tray drop rate and seedling tray transportation speed models were 0.9977 and 0.9921, respectively, both close to 1. There were the variations of 0.23% and 0.79%, respectively, indicating a better fit and high prediction. The optimal working parameters were obtained after parameter optimization: conveyor belt speed 0.32m/s, motor power 350 W, where the transportation speed of the seedling tray retraction and extension device was 5.0 m/min, and the transportation speed of the handling device was 10.0 m/min. The theoretical seedling tray drop rate was 0.820% under the optimal conditions, and the seedling tray seedling tray transportation speed was 9.58m/min. The actual drop rate of the seedling trays was 0.833% in the verification test, and the seedling tray transportation speed of the seedling trays was 9.75 m/min, where the relative errors were 1.59% and 0.46%, respectively. The reliable prediction of the improved model fully met the agronomic requirements for the harvesting and release of seedling trays. The bidirectional conveying device for automaticretraction and extension bidirectional conveying device of seedling trays in solar greenhouses developed in this paper provides technical support for achieving rapid, efficient, and labor-saving intelligent conveying of factory-grown seedling trays.