The focal plane of a space camera will defocus under the launching vibration and shock and complex environmental conditions in space which are changeable. In order to compensate defocusing for focusing mechanism that is long array focal plane with heavy load, a focusing mechanism was designed. This focusing mechanism was driven by two sets of motion mechanism that could provide large driving torque. The two overconstrained lead rails could meet the need of mechanical property. Every mechanism contained a stepper motor, a screw, a encoder, a rolling guide and gear transmission. The finite-element emulation analysis method was used to establish simulation model of focusing mechanism. Through the modal analysis, it verified that this mechanism had a better rigidity and could meet dynamic requirement. Analysis of focusing precision and stability accuracy indicate that the focusing precision of focusing mechanism is 3.8 μm, the stability is less than 5″ and the synchronization precision is 1.1 μm. The design analysis and experimental results show that the focusing mechanism has high focusing precision and high reliability, which can make use of precision adjustment of long focal plane.