In this paper, a new type of focusing mechanism for space telescope is designed to compensate defocusing caused by alteration of space condition in orbit. The focusing mechanism is driven by a stepping motor and a worm gear reducer, transmitted by a ball screw and output by a flexible hinge lever. It has advantages of low mass, small overall size and high accuracy that is better than 1 mu m. As one of main parts of focusing mechanism, the flexible hinge lever is mounted on a fixed bracket and connected to the nuts of ball screw kinematic pair which has mechanical end stops. The focusing shaft attached to the flexible hinge lever is guided by two thin film boards to ensure the precise displacement along optical axis direction. The secondary mirror frame is connected to the shaft to complete focusing. Firstly, the focusing mechanism is designed. Then a modal analysis with finite element method for the mechanism is completed. The results show that the dynamic stiffness is high enough and meets the requirement.