Abstract: Deep application of organic fertilizer in orchards was often confined to the excessive concentration of fertilizer, and low utilization rate in traditional agriculture. In this study, a feasible layered backfill device was designed for the mixed soil and fertilizer. An auger was used to transport the backfill soil and fertilizer. Paddles were arranged to evenly mix the soil and fertilizer. Three groups of mixed backfill components were set to realize the layered backfill. A theoretical calculation was conducted to determine the parameters of the backfill auger, according to the amount of backfill needed to be transported. Specifically, the outer diameter of the auger was 0.280 m, the pitch was 0.200 m, and the rotational speed was more than 98 r/min. Taking the sideward angle of paddle, pitch angle of paddle and rotational speed as test factors, the relative standard deviation of organic fertilizer proportion and the average normal contact force of particle structure as evaluation indexes, a response surface simulation test was carried out to obtain the regression equation. An optimal combination of mixing paddle parameters was obtained, where the minimum evaluation index was taken as the objective. The best mixing effect was achieved, where the relative standard deviation was 40.35%, and the normal contact force was 0.33 N, when the sideward angle of paddle, pitch angle of paddle and rotational speed were 90°, 60°, and 200 r/min, respectively. An attempt was also made to explore the influence of the length for the paddle arrangement area on particle mixing in the simulation test. The results showed that the uniformity of particle mixing increased with the increase in the length of the paddle arrangement area, which tended to be stable at 400 mm. The mixing paddles were arranged in the whole length of the front and middle mixing backfill parts, and only within the length of 400 mm for the post-mixing backfill part. A deflector was also designed for a better stratification effect. The reason was that the soil and fertilizer cannot be evenly distributed in the whole width range, even piled up directly in one place, particularly for the wide fertilizing ditch. A bench test was performed on the movement of particles at the discharging port, thereby evaluating the mixing performance of the device. When the optimal inclination angles of deflectors at the front, middle and back discharging port were 70°, 65°, and 50°, respectively, the soil and fertilizer in each layer was evenly distributed in the whole width of the fertilization ditch, indicating an improved stratification effect. A fertilizing ditch was opened with a width of 0.3 m, a depth of 0.4 m, and a length of 15 m. On the left side of the ditch, a ridge with a width of 0.2 m and a height of 0.2 m was set, on which the organic fertilizer was arranged. During the test, the feeding area was aligned with the ridge, and the discharging port was aligned with the ditch. After the test, three areas with a length of 0.5 m and width of 0.3 m were selected in the ditch, where 5-point sampling was used for sampling. The samples were laid flat in a box with a length of 280 mm and a width of 220 mm. Then the images were captured. An image processing was used to calculate the proportion of organic fertilizer. The relative standard deviation was 18.03%. In order to verify the accuracy of discrete element model and the reliability of the simulation optimization results, the sample size was set to be consistent with the bench test in the simulation test with the optimal paddle parameters, and the relative standard deviation of organic fertilizer proportion was calculated, which was 18.01%, the relative error was 0.11%. The results showed that the discrete element model was accurate, the simulation optimization result was reliable, and the mixing performance of the device was good, which can meet the requirements of soil fertilizer mixing operation.