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A new type of sub-soiling and rotary tilling unit for middle-power tractor was developed. The development of the unit has been based on the matching theory of tractor farm implements. Calculation was made with the parameter of power consumption per unit cultivated land area. Dynamic analysis, experiments of sub-soiling separately, rotary tilling separately and the combination work of the two were carried out. The experiments showed sub-soiling disturbance influence width matches rotary tilling width, fluctuation range was small during rotary tilling work, deflective torque has no influence on tractor driving stability basically.