The mobile lidar of USTC implements the continuous wind measurement in 15-60 km neutral atmosphere at night. Since the signal to noise radio was limited by background light during daytime operation, an ultra-narrow filter which combined an interference filter with a solid FP etalon was designed to reduce the background noise. An approach to solve the optimum parameters for the F-P etalon was proposed, based on the Frequency Discriminator, an etalon filter with bandwidth of 8.4 pm, center wavelength of 354.73 nm, free spectral range of 150 pm and peak transmission of 0.67 was achieved to decrease the background noise to one in eighteen. The improvement of SNR and velocity errors were simulated. The effect of temperature and tilting angle on bandwidth, center wavelength and transmissivity was calculated. A method of tuning by tilting angle was designed. The experiment result is highly uniform with the theoretical value.