Abstract: Low accessibility of large ditching equipment cannot fully meet the narrow space in grape vineyards in southern China. Manual ditching is also low effectiveness and efficiency. Fortunately, a 1K-50 orchard ditching machine has been developed in this case. Nevertheless, some challenges still remained regarding the stability of trench depth, high power consumption, and severe vibration of the machine during ditching. One of the key components in the ditching machine, the blade roller can encounter significant resistance, as the blades make direct contact with the soil during operation. This resistance has posed a strong impact on the ditching power and vibration of the machine. It is very necessary to analyze the ditching power, working resistance, and soil movement during deep cultivation with a reverse-rotary ditching machine. The discrete element method (DEM) can be expected to explore the ditching power and vibration. Compared with the theoretical calculations and agricultural machinery experiments, the interaction between the blade roller and the soil can be determined to optimize the blade roller parameters and the soil flow state from a microscopic perspective. At the same time, efficiency can also be improved to reduce research costs. However, the number of high-magnitude particles can be limited to establish the soil particle model in a 1:1 physical scale, when discrete element software runs the large-scale agricultural machinery simulations. Coarsening can be used to simplify a complex particle model into a larger-scale particle combination without considering the relatively minor physical phenomena. System details can be removed to significantly reduce the computational load. Particle size, shape, and contact properties between particles are required for the key factors to consider in the coarsening process of the model. This study aims to clarify the influence of the scale ratio on the ditching power, working resistance, and soil movement state of the machine, in order to simplify the issue while considering the key factors. Taking the ditching component of the 1K-50 ditching machine as the research object, EDEM software was used to construct a blade roller-soil model suitable for the soil environment of grape vineyards in southern China. The simulated ditching test was conducted under the working conditions of a cutter roll speed of 132 r/min, forward speed of 0.06 m/s, and ditching depth of 0.3 m. The results show that the average ditching power, horizontal resistance, and vertical resistance were 3.73 kW, 923.85 N, and 148.30 N, respectively, in the original particle state during the stable operating stage of the blade roller. The error of simulated and actual ditching power was 9.9%. The enlarging diameter of soil particles caused the variation in the ditching power of the blade roller, working resistance, and soil movement state. Specifically, the average ditching power and horizontal resistance decreased with the increase in the scale ratio, while the average vertical resistance decreased first, then increased, and finally decreased again. The throwing speed decreased, and the shape of the thrown soil was loose at the advanced moment when the soil was thrown out from the forward section of the blade. But there was no change in the order of the soil distribution in the soil layers at different depths. The simulation time was reduced by more than 99% when the scale ratio was between 2 and 5. Nevertheless, the error exceeded 32% in the ditching power of the blade roller simulation, compared with the actual. Once the scale ratio was 1.2, the relative error of the model in the ditching power was kept at 11.1%, with a simulation time of 22.9 h and a simulation data volume of 313.72 GB. The findings can provide a strong reference for the discrete element coarsening in the field of agricultural machinery. A theoretical basis was also offered to construct a coarsened model of the blade roller-soil system.