The drilling pressure control model was established based on the structural characters of full hydraulic driving head ground coalbed gas drilling rig, feed mechanism mathematical models and drilling pressure control experience formulae; the fuzzy PID controller was designed and simulation analysis was carried out. By adjusting the coefficients value of threshold pressure value and rock drillability value, the influence of the different strata and the parameters of controlled objects in drilling courses on auto-drilling system were simulated and the simulation analysis indicates that this system has good adaptability and anti-interference ability with high response speed and small overshoot. Therefore, it is feasible to realize constant drilling pressure by fuzzy control, which has guiding value for automation and intelligent control of drilling machine.