A method for managing key in Multimedia Broadcast/Multicast service comprising steps of defining a valid MTK ID interval for each generated MSK and sends it to a UE along with a MSK by a BMSC; after receiving the MSK, saving a valid MTK ID interval of the MSK by the UE; and defining a MTK ID for each generated MTK encrypted with the MSK and sending the MTK ID and the MTK to the UE after encrypting them with the MSK by the BMSC This MSK is valid only when the transmission of the MTK within MTK ID interval is in operation. Therefore, once the UE finds out that some newly received MTK's MTK ID is beyond said MTK ID, it deletes the MSK that is applied in said MTK transmission's encryption correspondingly.