The synchronous motion accuracy at both ends of focal plane of a focusing mechanism in a space camera was analyzed, for it was difficult to be eliminated and would affect on the focusing accuracy of the camera directly. The characteristics of the focusing mechanism of the camera with a long focal plane were researched, then it pointed out that the synchronous motion error at both ends of focal plane is the major factor that effects on the focusing accuracy of the camera. By analyzing main error sources of synchronous motion error, a mathematical model was established, and Monte Carlo Method was used to simulate the error model. Finally, the synchronous motion accuracy was tested to verify the validity of the simulation result. Simulation results indicate that synchronous motion accuracy is 0.02 mm,which is coincident with the test data well. It is concluded that the method can estimate accurately the synchronous motion accuracy, and can support the determination of focusing strategy on orbit.