In accordance with the characteristics of a high resolution space remote sensing camera, a focusing mechanism is designed to compensate for defocusing. The accuracy of focusing mechanism is analyzed from the transmission mechanism, the photoelectric encoder and the focusing control system. Before calculating the total errors of the focusing mechanism, all of the causes and values of error are discussed. After testing the accuracy of the focusing mechanism with closed-loop control mode, the data of the encoder codes and the focusing mirror displacements are processed respectively. Contrasting the results, the accuracy of the focusing mechanism is obtained, equal to ±3.1127 μm, which is in accordance with the theoretical value, and it proves that the design of the focusing mechanism is effective. Finally, the reason of differences between the accuracy results of the encoder codes and the focusing mirror displacements is obtained after analyzing the testing method. The experiments show that the accuracy of the focusing mechanism meets the camera′s accuracy requirement of ±10 μm.