The invention aims at providing a method for defending a DDoS aggression in a P2P media stream system, which includes the following steps: the step that the node sends a joining requirement information to the media stream server is included and in the step, after receiving the joining requirement information, the server informs the joining requirement information of the node to the reputation node and the controlling node and return the partial nodes and a initial reputation value; a step that the reputation nodes calculate the node reputation value is included and in the step, after receiving the notices, the reputation node and the controlling node store the information of the node into own node list in order to evaluate and control the action of the node; the step that the node requires a data from the media server or local network is included, which is used for requesting the needed media data by the node; a step that the controlling node judges the node requirement rate is included, which is used for calculating the node requirement rate; a step that whether the requirement rate exceeds the limiting value is judged is also included, which is used for judging whether the requirement rate of the requirement node exceeds the limiting value or not; if the rate exceeds the limiting value, the requirement is stopped, otherwise, the request of the node is permitted.