The principle of wind measurement for a direct detection Doppler wind lidar developed with a Fabry-Perot etalon is described. The measurement error of the system is affected by the receiving beam divergence angle. The numerical calculation for the Doppler wind lidar system has shown that the percentage error is increased slowly to 5% for the beam divergence less than 1mrad. The analysis results for the measured beam divergence angle indicate that the error in the velocity dynamic range is less than 0.6m/s up to 5 km.