Abstract: The major way of heat rejection from spacecraft to space was heat radiation whose heat release was proportionate to fourth power of the temperature of the heat rejection. Regarding that,a concept of heat pump used to strengthen rejecting heat was proposed. The mathematic model of the heat pump system for spacecraft is built and we analysis the parameters,such as the condensing temperature,degree of supercooling,refrigerants,influencing quality of the heat pump system.The results indicate that the condensing temperature and degree of supercooling are vital for lightening the system mass and they have the optimum value for the system. Systems use different refrigerants have different performance and the thermal control system which uses heat pump is better than those without heat pump system in lightening the system mass.