The theory and method of measuring moving target′s velocity using atomic transmission are described. A diode pumped frequency doubling CW Nd∶YAG laser has been used to demonstrate the feasibility of the method. An iodine filter with wing width of 380 MHz is used in the experiment which represents a velocity range of 0～50 m/s. The laser frequency is locked on the center of the slope of transmission of another iodine filter. The experimental error as small as 30 cm/s is obtained. The result and experimental error are analyzed. This method can be applied to lidar for measuring the atmosphere wind.