A Fabry-Perot etalon with high spectral resolution is used as the frequency discriminator in the direct detection wind lidar.The laser frequency and the spacing of cavity drift as the temperature changes,which will cause the increasing of error.A frequency tracking solution is proposed.A backscatter fiber is used to expand the width of laser pulse.A locking filter is added to dual F-P etalon.Photo Multiplier Tube (PMT) is used to detect the intensity of light and a frequency tracking circuitry is designed.The results of simulation experiment indicated that,the spacing of cavity is able to lock to the laser by this method.