Abstract: The transmission change of ZnSe/MgF2/K9 filter induced by CW laser(1.06μm) is analyzed in this paper. At room temperature, the change of the film ' s refractive index induced by heating is measured with the detecting laser beam (0632 8μm). As a result of this change, the filter's transmission varies nonlinearly. When laser beam of different power with the spot diameter 0.75mm irradiated on the ZnSe/MgF2/K9 filter, temperature change responding to irradiation time is measured. Typical experiment results are as follows : The film temperature damage threshold is 90℃ with laser power 30W and irradiation time 2.52s, checked surface of the film is formed with irradiation time 10s.