Abstract: The operational state of the threshing and separation unit in a combine harvester directly affects harvesting efficiency and grain quality, yet real-time monitoring of the material separation process remains challenging due to field vibration interference and signal delay in existing sensors. To validate a newly developed impact-based piezoelectric sensor and acquire representative impact signals for fault diagnosis, this study proposes an ‘impact-throw’ dynamic similarity principle and develops a modular simulation test bench with precise parameter controllability and material recyclability. The bench comprises a hopper for quantitative feeding, an adjustable inclined slide rail for directional dispersion, and a segmented-blade impeller (10 blades, each divided into three sub-blades with 12° phase angle) for controlled throwing, enabling high-fidelity simulation of localized separation at the front end of the threshing drum. A discrete element model was established using EDEM 2021 R2 with calibrated parameters for rice grains and straw. A full-factorial experiment was conducted with three factors at three levels: slide rail angle (45°, 50°, 55°), impeller speed (400, 500, 600 r/min), and horizontal distance between slide rail and impeller axis (−10, 0, 10 mm). Four evaluation metrics were adopted: material separation velocity, regional load distribution (RLD), separation trajectory, and total difference index (TDI). Analysis of variance revealed that impeller speed dominated separation velocity (P<0.01, F=164.101, R²=0.979), horizontal distance governed separation trajectory (P<0.01, F=1531.028, R²=0.998), and slide rail angle primarily regulated RLD (P<0.01, F=158.297, R²=0.977). The optimal parameter combination (impeller speed 500 r/min, slide rail angle 50°, horizontal distance 0 mm) achieved a TDI of 0.019, with a separation velocity of 2.937 m/s and an RLD of 0.181, closely matching the target values. Using this validated bench, the effect of sensor impact plate installation on material distribution uniformity was evaluated at four preset positions (S1: L1×D1, S2: L2×D1, S3: L3×D1, S4: L4×D6) and their combinations. With the jitter plate vibrating at 5 Hz with 5 mm horizontal and 10 mm longitudinal amplitudes, the dual-plate configuration at S1 and S2 reduced the weighted comprehensive distribution index by 28.86% compared to the optimal single-plate configuration (S3), decreasing from 1.05 to 0.747, and reduced the population standard deviation by 26.76% compared to the no-impact-plate condition, from 2.13 to 1.56. These results confirm that installing the sensor impact plate at appropriate positions not only avoids disturbing material flow but actively improves distribution uniformity, thereby supporting enhanced cleaning efficiency. The developed test bench provides a reliable and parameter-controllable platform for validating separation state monitoring sensors and acquiring impact signals for threshing drum fault diagnosis.