In this paper we analyze characteristics of the ground reaction force (GRF) experienced by the legs of the quadruped robot during stance phase in walk gait, in particular, when the height of center of gravity (COG) of the quadruped robot is changeable. We also build the dynamics model of the quadruped robot. Two dynamics equations during swing phase 

and stance phase are established, respectively. Additionally, we design a controller to adjust the height of COG of the quadruped robot. The controller uses the central pattern 

generator (CPG) model to generate basic rhythmic motion, and utilizes the discrete tracking differentiator (TD) to implement the transition between two different rhythmic medium values of the CPG. The combination of the CPG model and the discrete TD enables the quadruped robot to adjust the height of COG according to the environment. The ground reaction peak force and the joint torque of the quadruped robot increase with the reduction of the height of COG. Finally, we give a simulation example and the results, including an analysis of the vertical reaction force and the joint torque of the quadruped robot.