Abstract: An identification method of loss factors of fiber/resin composite under thermal environment was presented. Firstly, the composite thin plate test-specimen of such material was taken as an example, and its vibration responses under thermal environment were theoretically solved based on the complex modulus method. Then, a laser scanning frame model of composite thin plate was established, and on the basis of the vibration responses obtained by laser scanning and complex modulus method, the relative error function of the responses was constructed by the least square method, so that the loss factors in the different fiber directions under thermal environment can be identified. Next, the identification principle of loss factors of fiber/resin composite under thermal environment was illustrated, and a reasonable and standard identification procedure was summarized. Finally, a vibration testing system of composite thin plate under thermal environment based on laser scanning technique was set up, and TC500 carbon fiber/resin composite thin plate was taken as a research object. The first 4 resonant responses were measured from the room temperature to 300℃, and consequently the loss factors were identified by the 1st resonant response data. It is found that when it raises from the room temperature to 300℃, the loss factors of fiber/resin composite material will increase gradually. In addition, the corresponding loss factor data at 100℃ was substituted into the theoretical model, and the 2nd, 3rd and 4th resonant response results were theoretically obtained at this temperature. By comparing these theoretical results with the experimental results, the relative errors are within the range of 1.4%-13.8%, so the effectiveness and practicability of such identification method have been verified.