In order to improve the quality and efficiency of the mechanical harvest of subterranean crops, and to solve the problems of high cost, low efficiency, inconvenient data collection and greatly affected by weather on the field test of harvest machines for crops under soil in China, an automatic alignment digging harvest test bench for crops under soil harvesting is designed in this paper. The test bench is mainly composed of a transmission device, speed adjustment device, off-centerline distance adjustment device, root fixed and plant spacing adjustment device, deviation detection device, hydraulic correction actuator and digging simulation device, measurement control system and hydraulic system. The overall structure and working principle of the test bench are briefly analyzed, and the structural design and parameter analysis of key components are especially introduced. A gantry with adjustable front and back distance and height between the deviation detection device and the digging simulation device is designed. Using four-bar linkage, a deviation detection device is designed to detect the distribution of subterranean crops in the soil and transfer it to the controller. An digging simulation device with resistance is designed to simulate the soil resistance of the digging device during the process of digging and harvesting. A device with root fixing, plant spacing and off-centerline distance adjustment is designed to simulate the actual plant spacing, off-centerline distance and other growth conditions in the field. The mathematical relationship between the expansion and contraction of the hydraulic cylinder and the moving distance of the digging mechanism is established. The hardware of the measurement and control system is mainly composed of control box, computer, pressure sensor, flow sensor, displacement sensor, speed sensor, hydraulic solenoid valve, hydraulic cylinder and high-speed camera. The software adopts the fixed dual-threshold dead-zone PD control algorithm. In order to improve the generality of the test bench, it is only to replace the original detecting rod with ridge shaped detecting rod and detecting wheel, and replace the root fixing device with ridge shaped simulating adjusting device when the peanut, potato and other crops completely buried in the soil are dug. Taking the beet harvest as the research object, the plant spacing, off-centerline distance and forward speed as the experimental factors, and the missed digging rate as the index, the comparative tests between the test bentch and field operation was carried out. The test results showed that the leakage rate of the test bentch is 2.33%-2.72%, and the leakage rate of the field test is 2.38%-2.92%. The test bentch has good excavation accuracy, the absolute deviation rate range is 2.10%-6.85%, the average deviation rate is 3.67%, and the larger the leakage rate is, the larger the deviation rate is. This study can provide reference for the design of automatic alignment digging harvest system of underground crops such as beet, radish, potato, etc.