In order to resolve the SPIT（Spam over Internet Telephony）security risk in the ALL-IP convergence network, a detection and prevention method based on feedback judgment is proposed in this paper. Introducing the participation of the users and combining the mechanism of trust and reputation can make the direct and indirect use of feedback simple effective and lossless. Improved inference algorithm embodies the distribution characteristic of SPIT behavior and reflects the weight relation of factors which influence the result. In addition, the incremental learning algorithm gives the Real-time trust and reputation. The algorithm dynamically integrates the trust with the reputation and makes a Comprehensive Evaluation of the SPIT. Experiment and analysis results show the better accuracy and sensitivity of the method, indicate the efficiency of the detection and prevention.