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In response to the requirements of soil-covering with large width, thin thickness and high uniformity for mechanized soil-straw layered mulching technique in orchards, a supporting soil-covering device of orchard straw mulching machine was developed, which mainly consisted of a soil-taking screw, two soil-throwing wheels, a soil-collecting trough, and a soil-covering screw. The soil-taking screw with width of 1.5m provided a flat foundation for straw layer laying, ensuring a flat-covering of straw layer and uniformity of the thin soil layer. The soil-throwing wheels on both sides of the soil-taking screw threw the soil back into the soil-collecting trough, avoiding the stuck problem of the device caused by foreign objects such as gravel. With discrete element simulation, it was determined that the terminal double helix structure was adopted for the soil-taking screw, the number of the soil-throwing wheel blades was 10, and the blade deflection angle of the soil-throwing wheel was 10° to improve the soil-covering uniformity and reduce the power consumption. The field tests showed that the device had a soil-covering width of 1.5m and a thickness of 20.8~31.7mm. The coefficient of determination of the regression model for the standard deviation of the soil-covering thickness was 0.9342 with error of 1.8%. The optimal combination of operating parameters with the standard deviation of the soil-covering thickness as the response value was decided as the rotation speed of the soil-throwing wheel was 300r/min, the soil-throwing angle was 40°, and the soil-taking depth was 20mm. Under the combination, the soil-covering thickness was 22.9mm with standard deviation of 3.7mm, which met the requirements of soil-covering with thin thickness and high uniformity. The research realized the mechanization of the orchard soil-straw layered mulching agronomy and provided a method and equipment for orchard management in arid areas.