Abstract: The transient analysis model for the 200kW Advanced Space Fast Reactor RAPID-L is developed. The unprotected transient overpower(UTOP) and unprotected loss of flow(ULOF) are analyzed with the model. It is demonstrated that the quick lithium expansion module (LEM) can response promptly according to the coolant temperature variation and safeguard the reactor without shut-down, so RAPID-L has the characteristic of inherent safety.