A security key generating method, device and system are provided, wherein, the method is used for generating a security key in the process of the handover to an EUTRAN network from other network, the method includes: an MME generates a security key based on a root key KASME of the EUTRAN network, a specific value and/or other parameters and transmits a handover request message carrying the security key to a target evolved Node B, i.e. eNB; a UE generates the security key which used by the target eNB based on the root key KASME of the EUTRAN network, the specific value and/or other parameters. The application of the present invention adopts a specific value, KASME and/or other parameters to output a security key, which can solve the problem existed in the related technology, the problem is that the middle security key used by access layer while handover between different access system can not be generated, and then it can not realize the security protection of the access layer. So the application of the present invention makes the signaling and/or data of the access layer be efficiently protected, thereby improving the security of the access layer.