Since current studies on NGN mainly focus on

passive defense, which has a certain limitation in such aspects as technique realization, the paper starts from a detailed analysis on security vulnerability of NGN, and

then discussed possible attacks by hijack, concluded NGN attacks, suggested the key active defense strategies in network-built security, boundary security, access security and information transmission security, and

presented the developing direction to be studied further,providing a good reference for the research on NGN.