A series of under-actuated robots is taken as the research object, and a mechanism for optimizing the end control of the robot is designed. Based on this, the kinematics model was established. The motion trajectory of the robot was determined to control the following of the end of the robot. For the purpose of the multi-joint fit, the end of the robot tool was enabled of flexible tracking of the surface. Experiments show that the robot designed can move according to the specified trajectory, and under the action of the passive axis, the robot end can maintain good flexible contact with the tracking surface.