The soil mulching device plays an important role in the sowing stage of crops. Compared with the traditional narrow film planting mode, under the ultra-wide film planting mode adopted by crops in arid areas, the soil transportation distance inside the soil mulching device increases, resulting in the problem of uneven soil coverage thickness between sowing rows, which seriously affects the crop emergence rate. There are many methods available to solve the problems of ultra-wide film planting, such as theoretical analysis and discrete element method (DEM) simulation. In this paper, we design a covering device consisting primarily of the soil disc and the cylindrical sealer. By constructing and analyzing a mechanical model of the soil mulching device, the parameters related to the working performance are determined. In order to further optimize the parameters, the simulation model of the device and soil operation process is established by using DEM software. We design a single-factor simulation test. The structural parameters and operating parameters of the cylindrical sealer are divided into two experimental groups, and the soil transport volume (STV) and the coefficient of variation of the soil coating cover uniformity (G1) are used as response values. Based on the parameters of the single-factor test after narrowing the optimization range, the response surface test is designed. According to the experimental results, a quadratic regression model of STV and G1 is established, and the influence of the interaction of each parameter is analyzed. Finally, the optimal values of the structural parameters and operating parameters of the soil mulching device are determined. The actual field test was completed. The soil coverage thickness was between 1.42 cm and 1.50 cm, and G1 was 2.59%. The test results were very satisfactory. This shows that it is reliable to optimize the parameters of the soil mulching device by theoretical analysis and the DEM.