A method for preventing IP address from unexpected dispersion when using point-to-point protocol is disclosed. The method comprises the steps: starting and initializing a PPP session; performing a PPP LCP negotiation, in which times of the successful PPP LCP negotiation is limited; performing a PPP authentication, in which an authentication request to AAA client is sent only once; performing a PPP IPCP negotiation, in which an accounting-start request is sent only once and times of the successful PPP IPCP negotiation is limited; and terminating the PPP session and releasing the IP address. The method of the invention for preventing IP address from unexpected dispersion when using point-to-point protocol effectively avoid excessive oscillation of a LCP state machine and a IPCP state machine by limiting times of the successful PPP LCP negotiation during the period of PPP LCP negotiation and times of the successful PPP IPCP negotiation. In the meantime, the method the invention effectively solves the problem of IP address unexpected dispersion by sending an authentication request to an AAA client only once during the PPP authentication phase and ensuring that the PPP session sends an accounting-start request only once.