A model as well as the methodology is proposed to analyze the cryogenic performance of space infrared optical payload. And the model is established from two aspects: imaging quality and background radiation. On the basis of finite element analysis, the deformation of optical surface in cryogenic environment is characterized by Zernike polynomials, and then, the varying pattern of MTF of space cryogenic optical payload could be concluded accordingly. Then from the theory of thermal radiative transfer, the temperature distribution and the deformation of the optical payload under the action of inertial load and thermal load are analyzed based on the finite element method, and the spontaneous radiation and scattering properties of the optical surface and shielding factors between the opto-mechanical structure are considered to establish the radiation calculation model. Furthermore, the cryogenic radiation characteristics of the space infrared optical payload are obtained by the radiation calculation model. Finally, experiments are conducted using an actual off-axis TMA space infrared optical payload. And the results indicate that the background radiation of the space infrared optical payload is decreased by 79% while 33% for MTF at the thermal control temperature of 240K. In this circumstance, the system background radiation is effectively suppressed and the detection sensitivity of the optical payload is improved as well, while the imaging quality is acceptable. The model proposed in this paper can be applied to the analyzing cryogenic properties of space infrared optical payload, and providing theoretical guidance for the design and application of the space cryogenic optical payload.