Abstract: The performance characteristic and startup process of advanced space fast reactor RAPID-L were demonstrated with the transient analysis model. The results show that the refuelled period of RAPIDL is 10 years, and the reactor power can be adjusted by primary flow rate control, and it can start automatically with LRM(Lithium Release Module) and LEM(Lithium Expansion Module). Therefore, RAPID-L can operate for a long time and achieve autooperation without operator. So it fulfils the demand of space operation.