Through deeply analyzing on the principle, mode, character of SIP DoS and the flooding attacks faced by SIP network, the prevention model to combine a dynamic threshold adjustment with real-time dynamic prevention for SIP flooding attacks was proposed. This model included logically chi-square traffic judgement model, cumulative statistics model and IP prevention model, among which chi-square traffic judgement module and cumulative statistics module were combined to dynamically adjust the threshold and detect SIP flooding attacks, and IP defense model dynamic prevented IP-based SIP flooding attacks. The experimental result shows that the model can effectively detect and prevent the SIP flooding attacks, and reduce the probability of SIP proxy server or IMS server been attacked when the SIP network is on the abnormity.