On the basis of the characteristics of optical systems in a space camera, a focusing mechanism suitable for the camera was designed optimally. Firstly, to meet the overall design requirements of the camera for the lower mass and smaller size, a double-slider mechanism combined a screw and a link was chosen to transform the screw's running into the line movement of the focusing mirror along the optical axis direction and to overcome the defocusing of the system by adjusting the image movement. Then, the focusing accuracy of the focusing mechanism was analyzed and the error source and error calculation were discussed. Finally, the vibration and accuracy tests were performed on the mechanism. It shows that the optimized focusing mechanism has a weight of only 2.5 kg and a shape size of 186 mm×144 mm×60 mm. Furthermore, the vibration and accuracy tests indicate that the focusing mechanism can offer the first frequency of 505 Hz, positioning accuracy of 0.008 mm, and the repeatability positioning accuracy of 0.002 5 mm, respectively. These results satisfy the requirement of the optical system very well and show that the mechanism is characterized by the compact structure, high stiffness and good accuracy.