Ubiquitous network is a kind of standard heterogeneous network. It is a hot research topic to secure switching between networks. This paper analyzes EAP-AKA, which is used during handoff across heterogeneous networks. However, this protocol has high authentication delay and is confronted with several security threats, such as user identity disclosure, man in middle attack and DoS attack. Moreover, access point of the access network is not verified, leaving the user under attack even after heavy authentication procedure. To deal with the above security vulnerabilities, an improved secure authentication protocol for ubiquitous network based on EAP-AKA protocol is proposed, extending the applicability of traditional EAP-AKA protocol from the 3G system to ubiquitous network. The new protocol reduces authentication delay and effectively protects identities of users and access points. In order to avoid main session key leakage, the Diffie Hellman algorithm is used to generate a symmetric key randomly each time. The mutual authentication between user endpoint and the home network is also achieved in new protocol. Experiments and analysis verifies effectiveness and efficiency of the proposed protocol.