The utility model relates to a rotary drilling rig power head system with a function of multistage torque control, in particular to a rotary drilling rig power head hydraulic system. The utility model aims at providing a power head system which has a simple structure, is convenient to operate and has the function of multistage torque control. The rotary drilling rig power head system with the function of multistage torque control comprises a power head motor and a reversing valve, wherein two working oil ports of the power head motor are respectively communicate with two working oil ports A and B of the reversing valve through a main pipeline, an oil inlet P of the reversing valve is communicated with a hydraulic power station, an oil return port T is communicated with an oil tank, wherein a multistage torque control device is also arranged between the main pipeline and the oil tank, and a plurality of selectable overflow pressures are arranged in the multistage torque control device. When the pressure of the main pipeline reaches the selected overflow pressure, the main pipeline and the oil tank are directly communicated by the multistage torque control device.