Principle of operation was analyzed for laser warning device based on Fabry-Perot etalon,the relations were deduced among swinging angle of F-P etalon,laser incident angle and refraction angle.The method was achieved to calculate laser incident angle.Take laser of 1.064μm wavelengh for example,the changing relations between transmission light intensity and swinging angle were calculated at 4 different incidence angle.Three different laser incident angles were measured by processing software.The results show that the method is valid in measuring laser incident angle.