The position of optical focal plane for a space remote sensing instrument will be changed in severe launching process and complex working thermal environments, which effects the imaging quality of the remote sensing instrument seriously. To compensate the displacement of the optical focal plane and to ensure good imaging quality consequently, this paper illustrated the design scheme for a focusing mechanism and analyzed its error sources aiming to the working environments and accuracy demands for an infrared camera. The effect of structure error on the system accuracy was analyzed and the precisions of different structures were classified. An error analysis model was established based on homogeneous transformation matrixes which described the relation between topology structure and coordinate transformation. In addition, the characteristics and distribution of error sources were analyzed in detail, and the relationship between matrix parameters was discussed. The results of numerical calculation by Monte Carlo method demonstrate that the design mechanism is reasonable and satisfies remote sensing requirements. The measurements show that the axial position accuracy of focusing mechanism is (-0.01±0.003 6) mm with a tilt less than 25″and a displacement of 0.005 mm through travel distance. The focusing mechanism has been implemented in the space flight.