When a space camera is effected by the changes of gravity,temperature,pressure in space environment,the image detector can not be matched with the focal plane for mirror position change. In order to improve the imaging quality,a straight-line focusing machanism driven by step motor and glided on liner guide is designed based on analysis of several kind of focusing methods and discussion on focusing mechanism.This mechanism uses ball screws to translate the rotation to the line movement, and select an absolute encoder to detect the displacement. It has the advantages of light friction and simple transmission chain and can reduce the transmission error effectively. The precision of this mechanism is checked by open-loop control and closed-loop control. Moreover,the mean value and standard deviation are calculated by statistics analysis, and the residual error curve is plotted based on the test data. The measuring results show that the focusing machanism has a high precision, the error of closed-loop control is 0.002 5 mm, and the error of open-loop control is 0.001 5 mm, which are less than the precision of permission.