In order to improve the yaw accuracy of wind turbines in actual operation, the accuracy of wind direction measurement by mechanical anemometer is studied by combining horizontal lidar test with numerical simulation of computational fluid dynamics (CFD). Through analyzing the test data and simulation results, the following conclusions are drawn: 1) When the speed of wind turbine impeller is less than rated speed, and with the increase of the incoming wind speed, the deviation of wind direction measured by the anemometer decreases gradually, and the quadratic function between the incoming wind speed and yaw angle can be used to correct the yaw deviation better. 2) When the rotating speed of the runner reaches the rated speed, and with the increase of the incoming wind speed, the deviation of wind direction measured by the anemometer remains near the constant of 3.54°. When the yaw error is corrected by a constant of 3.54°. 3) The load of the unit is reduced and the generating capacity can be increased by about 1% after the yaw error is corrected by a piecewise function.