A method for negotiating a security capability when a terminal moves is provided. When a user equipment (UE) moves from a second/third generation (2G/3G) network to a long term evolution (LTE) network, the method includes the following steps. A mobility management entity (MME) acquires a non-access signaling (NAS) security algorithm supported by the UE, and an authentication vector-related key or a root key derived according to the authentication vector-related key, selects an NAS security algorithm, derives an NAS protection key according to the authentication vector-related key or the root key, and sends a message carrying the selected NAS security algorithm to the UE. The UE derives an NAS protection key according to an authentication vector-related key thereof. A system for negotiating a security capability when a terminal moves, a UE, and an MME are further provided.