Abstract: In order to solve the problem of signal-to-noise ratio decreasing with the decreasing of cavity length during the miniaturization of ring laser, narrow band filter was designed for suppressing the discharge glow of He-Ne gas without affecting on laser output. Based the unique step automatic coating design method of OpenFilters software and using the initial film thickness as the clue, a number of non-structured membrane structures were designed for the requirements of practical application of spectral characteristics. An optimized narrow band filter for crystal monitoring was selected under the consideration of layer number, thickness and stress. When s polarization light was incident from the air at angle of 45, the filter had peak transmittance of 99.9% at central wavelength of 632.8nm, full width at half maximum of 5.2nm and average transmittance at cutoff band of 1.35%. The results show that the filter can suppress the discharge glow of He-Ne gas effectively and improve the signal-to-noise ratio of miniaturized ring laser.