Abstract: For the problems that frequent start-stop of current drilling rig leads to safety accidents, such as collapse of hole wall of the coal, damage and loss of pipes, unavailable bore and so on, this paper introduced the former power head device of the gas drilling rig, the core device was the mechanical chuck which could rely on the hydraulic motor to automatically clamp and loosen rig without stopping drilling. This paper especially focuses on the design scheme, working process of clamping and loosening, and studies dynamic clamping force in detail. The results of mathematical analysis in Simulink show that the friction coefficient has a great influence on the clamping force, and reducing friction coefficient is helpful to increase clamping force.