Abstract: When a space thermionic reactor is in an unexpected dropping accident, reactivity control measures must be taken to ensure the reactor is in the subcritical state. SPACE-R which is designed for 40 kWe, and 10 years of life is used for supplying space electric power. There are two control measures which are proper for unexpected dropping accident of SPACE-R: Adding neutron absorber in the fuel cavity and adding burnable poison robs in the moderator. By calculating reactivity of SPACE-R using MCNP code, there is a conclusion: The two measures are all useful for controlling reactivity of SPACE-R in the unexpected dropping accident. Furthermore, there is a final design scheme which meets the critical safety criteria for the unexpected dropping accident.