Based on analyzing the structure of omni-directional laser warning system,the contradiction between wide field of view and the narrow-band spectrum filter is expounded.The Fabry-Perot model is set up. And the transmissivity of the filter vary to different incident angle is analyzed from the factors such as reflectivity of the reflective coating,phase jump and optical path difference.The relationship formula of incident angle and the transmissivity is given out, and its validity is verified by experiment.