In order to improve the ground pixel resolution and positioning accuracy of ground targets,the focusing control systems should be designed to compensate the defocusing of the CCD plane in a tridimentional mapping camera. According to the optical structure and focal depth of the camera,a focusing mechanism based on a cam is designed,and focusing schemes and focusing quantity are confirmed.Moreover,the reliability design and a removing return stroke error method is used to guarantee the strength and rigidity of the mechemism and to improve the focusing accuracy. Analysis of transmission error and validated tests indicate that the error of this focusing mechanism is ±1.71 μm;linearity of CCD plane is 11.2″ in the X direction and 11.8″ in the Y direction;and the circumaxis angle of CCD plane is 4.7″.The result of precision analysis shows that the focusing mechanism can meet the requirements of the users.