With the development of deep space exploration, the energy requirements of spacecraft are increasing. How to provide more reliable and lasting energy is a complex problem. The space reactor offers outstanding advantages, such as good environmental adaptability, wide power coverage, compact structure, and strong endurance. Therefore, space reactors have broad application prospects in space exploration.The work of this paper has developed a transient coupling code for the space thermionic reactor TOPAZ-II based on OpenFOAM. The reactor neutron physics model, transient heat transfer model, thermoelectric conversion model, and electromagnetic pump model are included in the developed code, considering the Doppler effect. The thermoelectric conversion model and the transient coupling code were validated by comparing them with the experimental values. The results show that the calculated results agree well with the experimental values.The start-up and shutdown of TOPAZ-II were simulated. The results show that the Doppler effect has a more significant influence at low temperatures. The change in fuel temperature relative to power is more gradual when the reactor is started than during shutdown. This is because thermal resistance impedes heat transfer between regions, and changes in thermal resistance between regions affect the rate of change in fuel temperature. A sharp variation in electrical power can be observed during both start-up and shutdown because of the thermoelectric conversion characteristics in the work region. The emitter temperature is more sensitive to electric power. Thermionic fuel starts supplying power to the electromagnetic pump at 2250 s during start-up, and the power supply to the electromagnetic pump stops at 1200 s during shutdown. In the later stages of start-up and shutdown, a peak in coolant temperature appears, which is caused by the relative change between mass flow rate and thermal power. The calculation results prove that TOPAZ-II has a sufficient safety margin during start-up and shutdown.