The infrared radiation characteristics of space object and its change regulation are the important information for object properties and on-orbit status estimate, which is of most significance for the application of space remote sensing detection. Ground-based infrared imaging detection system, equipped with the atmospheric synchronous correction system, can effectively obtain infrared radiation characteristics of space objects. However, due to its long distance away from the ground sensor, space object presents point source features for the imaging system, thus it produces diffusion on the focal plane, which results in serious distortion of radiance information characterized by each pixel and optical signals aliasing between the target and the background. Therefore, significant error is introduced in the retrieval result. According to ground-based infrared imaging detection system, a retrieval method of infrared radiance of space point source target is proposed. By analyzing the radiative transfer from target to image plane and the imaging process, a calculation method of space point source target radiation is derived, which takes energy diffusion, pixel signals aliasing, FPA duty cycle and other factors into consideration. By the ground-based experiment of measuring the infrared standard stars, the inversion results of standard stars irradiance are compared to their nominal values, and the accuracy is better than 20%. The results show that this method can provide an effective way for acquisition of infrared radiation characteristics of space point source target.