Mobile IP is one of the key technologies to implement mobile computing. But its security still needs furtherresearch. After an introduction to mobile IP, IPSec and the threat to mobile IP security,an IPSec-based mobile IP se-curity architecture is presented in the article. This architecture combines IPSec and some other secure measures withmobile IP so as to give mobile IP stronger security. In the article this architecture is discussed in detail and a methodis proposed to traverse firewall using IKE.