Abstract: Abstract: Mechanical weeding is considered as an essential component in organic farming. It takes advantage of the features that crop seedlings have better developed root system whose distribution is broader and deeper than young weeds during intra-row mechanical weeding in farmland. According to above characteristics, young weeds between seedlings can be removed by claw-tooth and other mechanical components that loosen or move the intra-row topsoil. So, the claw-tooth loosening depth has great influence on mechanical weeding performance. However, existing intra-row mechanical weeders still have problem of poor working depth control due to their claw-tooth depth into earth controlled by the depth wheel walking between the crop rows. It is impossible for claw-tooth to be kept at stable loosening depth into earth during intra-row mechanical weeding, because of the variations in soil properties, including bulk density, moisture content and surface topography, which can lead to working depth variations. So the poor working depth control is one of the main reasons to high injury rate of seedlings and poor weeding effect during intra-row mechanical weeding. To improve depth control, an automatic control system, consisting of hydraulic system, regulating mechanism, ultrasonic distance measuring module, control unit and driving plate, was developed, which converts the difference between the desired spacing and the spacing measured by the ultrasonic distance measuring module into an input signal that is sent to the solenoid valve of hydraulic system. This will result in an extension or contraction movement of the hydraulic cylinder in the regulating mechanism, which changes the relative position of the weeding claw-tooth with respect to the soil surface and thus also its loosening depth and keeps the parts of soybean intra-raw mechanical weeding unit at a constant penetration depth. The aim of this work was to develop and evaluate the automatic control system based on ultrasonic distance measurement, which can be used to improve the working depth control of intra-row mechanical weeders. On the basis of research and development of comb-type intra-row weeding mechanism, an automatic control system for loosening depth of weeding parts was designed. A mathematical model of a hydraulic control system for the weeding parts' depth adjustment was established. Simulation and PID (proportion, integration, differentiation) emendation were carried out in MATLAB/Simulink based on the established transfer function. The simulation results showed that the steady-state response time was 0.48 s, and the static error range was 0.06-0.09 mm when the PID control algorithm was used to track control the desired loosening depth. And test and control test-bed research of the loosening depth for intra-row weeding parts based on the automatic control system were carried out. The experimental results of depth measurement showed that the control system was able to detect the depth in motion, and was not affected by the seedlings, grass and soil characteristics on the surface of the earth. At the working speed of 0.278, 0.556 and 0.833 m/s, the average contrast errors of measuring values were 4.95, 5.36 and 5.90 mm respectively, which were obtained by measurement with HC-SR04 ultrasonic module and manual measurement for the location of each sample point, and the maximum contrast errors were 6.6, 7.4 and 8.3 mm respectively. The experimental results of depth control showed that the stability control in loosening depth for weeding parts of intra-row mechanical weeding can be achieved. At the soil bin traveling speed of 0.278 m/s, the loosening depth variation of weeding parts could be controlled in the range of (30±8) mm, which could meet the deep control requirement of intra-row weeding. The research provides solution and idea to solve the problem of stable controlling in loosening depth of intra-row weeding parts, and also provides reference for research and development of soybean intra-row mechanical weeder with automatic control in depth and good working performance.