Inner heat cycle of designed kW fast axial flow CO 2 laser is discussed, and the heat balance equation is eastablished. Technical key of heat exchanger for this kind of laser is given, and compared with several other kinds of heat exchangers. Rectangle fin circle tube heat exchanger is chosen as the laser′s radiator, and its conducting heat is calculated. The result indicates that the rectangle fin circle tube heat exchanger which has been chosen is suitable to our kW fast axial flow laser.