The Fabry-Perot etalon is usually used for dispersing light in the receiver of the Fringe Technique Doppler Lidar for its high precision of analyzing optical spectrum. However, the paper describes a new method that a Fabry-Perot etalon combined with an aperture diaphragm can be used as a narrow-band filter of transmitted laser beam. Only when incidence of light is perpendicular to the plates of the F-P etalon, a whole number of wavelengths fits into one round trip of the cavity mλ=2d and the period of frequency Δv=k.c/(2d) or the period of wavelength Δλ=k.λ2/(2d) is k times FSR of the etalon, the spectrum can be transmitted through the filter. When these spectrum enter the F-P etalon again, the mth interference circular fringes of λ coincide with the (m∓k)th
interference circular fringes of λ∓kΔλ on the focus plane of convergent lens. Comparing the circular fringes of laser beam filtered with those of unfiltered, experiment shows that quality of light beam filtered is improved obviously. So this
technique could be useful for sensing atmospheric wind velocity.