Frequency stability of Q-switched Nd:YAG laser significantly affects the wind measurement accuracy of direct detection Doppler Lidar. The typical free frequency drift of diode-pumped Nd:YAG laser working at doubling frequency 532 nm based on seed injection can reach 15 MHz/min, correspondingly to a weed speed measurement error of 4 m/s. By using virtual instrument control technology of Labview, the frequency of pulsed laser is successfully scanned, and iodine absorption line 1109 is matched automatically, which get the frequency on one of the two slopes of the line. Standard deviation of frequency stability is about 0.8 MHz for a long time (more than 2 h), while the equivalent wind speed error is 0.2 m/s. The requirement of frequency stability is achieved during long-term lidar measurements.