The principle of wind measurement for a direct detection Doppler wind lidar with a Fabry2Perot etalon is described in the paper. The Doppler measurement error as well as the designed system parameters are affected by the beam divergence to the etalon. The numerical calculation for a designed Doppler lidar system has shown that the per2 centage error is increased slowly to 6 % for the beam divergence less than lmrad , and quickly to about 40 % at 2mrad.