In order to meet the variable speed drive requirements of the hydraulic motor of excavators under com-plex operation conditions,a variable speed control system with a shift valve was designed for the hydraulic motor.Then,the system model of the hydraulic motor was established,and the effects of some main parameters,such asthe diameter of the damper orifice at the pilot pressure port,the initial displacement of the valve spool,the diame-ter of the damper orifice at the valve pool and the spring stiffness,on the speed- switching stability,were analyzed.Theoretical and experimental results show that (1) setting a damper orifice at the pilot pressure port helps to im-prove the commutation performance of the shift valve and shorten the speed- switching time of the motor; (2) the ini-tial displacement of the valve spool,the diameter of the damper orifice and the spring stiffness all remarkably influ-ence the speed- switching stability of the hydraulic motor; and (3) the calculated results obtained by the theoreticalmodel accord well with the experimental ones.