Abstract: FSP (fission surface power) is a kind of space reactor fully investigated and demonstrated by the United States. To investigate the transient characteristics of the FSP system, the components of the FSP system were modeled and a related program was developed. The program simulated the steady state operating conditions of the system. The results show that the steady state calculation results are in good agreement with design values. A reactivity insertion transient and primary pump restart after stop transient were simulated. The calculation results are reasonable in trends, and it proves that the method and idea of modeling with the models themselves are correct. The results show that the oscillation of system’s power and temperature after a positive reactivity insertion is related with the different speeds of temperature change of loops in the system, also the smaller the interval between primary pump stop and restart, the better. This program provides a reference for the transient analysis and safety analysis of reactor system similar to the FSP system.