A security method in a wireless communication system is provided, which is used for providing a plurality of security associations between a user equipment, a relay node, and a base station node in a wireless communication system. The user equipment authenticates with a serving gateway in the wireless communication system through the relay node, such that a security association between the user equipment and the relay node is established correspondingly. The relay node establishes a second security association between the relay node and the base station node through the base station node.