In space-based infrared detecting backgrounds， the presence of clouds would cause serious interference to target detection， making it impossible to continuously detect and track targets and reducing the system’s detecting efficiency. For space-based detecting systems operating in starring mode， the background changes little over a period. It is feasible to analyze the detection system efficiency based on modeling background characteristics. Theoretical analysis， image simulation and analytical modeling methods are used together to perform a fast evaluation of space-based infrared platform availability under complex cloud backgrounds. Firstly， a preliminary evaluation model of space-based detection availability is established through analysis of the signal to clutter ratio when the target is crossing clouds. Factors influencing the detecting availability are determined through analysis. Then， a linear relationship is given between the frame numbers of losing parameters of the target and cloud based on simulated results. A theoretical model of availability is built up by analyzing the physical meaning of each parameter and the coefficient values are determined by regression analysis. Finally， the space-based detecting availabilities of the simulation， preliminary evaluation model， and theoretical model are compared under new simulating conditions， which verifies the correctness and robustness of the theoretical. The model in this work could be used as a reference value for evaluating the space-based optical detection system’s availability.