Abstract: For an automatic vertical drilling system, it is difficult to accurately control the anti-deviation azimuth downhole. Therefore, a study was conducted on the continuous control over downhole anti-deviation force of the drilling tool. The dynamical model of the mechanical-hydraulic system for the anti-deviation mechanism was established, without changing the tool structure, component and parameters. Using the model, the function relationship between the on-off time of the two-position two-way solenoid valve in a moment of one revolution of the drill pipe, the amplitude of the output anti-deviation force of the mechanism and the rotary speed of the drill pipe was derived. To obtain an expected anti-deviation force output by the anti-deviation mechanism, the duration time Δt for switching off the solenoid valve was calculated by the target anti-deviation force, and the pulse width cycle T that determines the Δt was determined by the rotary speed of drill pipe. Accordingly, the target control pressure of the hydraulic cylinder was respectively stabilized at 7.0 MPa, 5.0 MPa and 3.5 MPa in the experiment when the rotary speed of the drill pipe was set to 65 r/min and the solenoid valve was switched off for 50 ms, 60 ms and 70 ms respectively in one PWM cycle. The results showed that the downhole anti-deviation force of the automatic vertical drilling system could be stabilized at an expected range by PWM control.