Abstract: Abstract: As the core technology of mechanized conservation tillage, no-tillage sowing is a new type of agricultural cultivation method popularized in recent years, it directly sows in the field where the straw and roots of the former crops are left, which not only reduces water evaporation and soil erosion, but also can ensure the germination and growth of seeds, which is conducive to water storage and conservation, increasing production and income, and at the same time improving soil fertility to some extent. However, after the harvest of the former crops without any collection and removal treatment, the "full amount of straw field" has become the new normal state of China's cultivation. The traditional no-tillage seeding equipments are prone to easily increase the workload of straw crushing device when it entering the soil and extinguishing stubble, which causes the blockage and jamming of straw conveying device. In order to solve this problem, the back-throwing type no-tillage straw smashing planter was taken as the research object in this paper, and an automatic control system based on off-ground height of the straw smashing device was developed. Torsional deformation acquisition device was created based on a double fan-shaped through hole metal detection disc and proximity switch, which was installed at both ends of the driving shaft to effectively monitor the rotation speed of the driving shaft and the stagger angle caused by the change of torque load. The design was based on the 32-bit ARM CortexTM-M3 core microprocessor hardware platform, which constituted the data acquisition and output drive unit of the automatic control system, the driving shaft rotation speed and stagger angle was collected in real time, the change trend was analysed through the system software, and current working conditions was identified. According to the corresponding output control signal, the straw height adjusting mechanism timely corrects the off-ground height of the straw smashing and conveying device to stabilize the working load. The straw height adjustment mechanism was composed of a solenoid valve, a hydraulic cylinder, a fixed bracket and a supporting ground wheel. The hydraulic cylinder's high pressure driving oil was taken directly from the working tractor to improve the applicability of the device. The performance verification test showed that under the driving shaft's rated rotation speed of 2 500 r/min, when the height of the straw crusher off the ground changed the working load, the automatic control system controled the rotation speed of the driving shaft between 2448 and 2632 r/min, the change of the stagger angle of the driving shaft was ± 0.002 4 rad, and the response delay time of the control signal to the change of the stagger angle was 0.24 s. The field operation comparison test showed that when the back-throwing type no-tillage straw smashing planter was equipped with automatic control system based on off-ground height of the straw smashing device, the passability of the machine was greatly improved, the occurrence of blockage was eliminated, the operation efficiency was increased by 52.9%, and the height of surface stubble was reduced by 43.4% after straw smashing operation. In conclusion, The design of this paper uses the change trend of the driving shaft rotation speed and stagger angle to identify the working condition, effectively eliminating the influence of mechanical structure parameters and material differences, and the mechanical structure is light and easy to install, which can provide reference for qualitative monitoring of torque load of agricultural machinery.