In order to reduce the illegal overspeed on highways and improve the traffic safety monitoring system, a portable laser Doppler velocity measuring system for long-distance work is proposed. The principle of laser Doppler velocimeter and the method of beam transformation are described in detail. And using the structure of the double lens telescope system, the laser velocimeter for highway overspeed detection is built. The results of theory and experiment show that this laser Doppler velocity measuring system has the characteristics of long working distance and large depth of field, and the vehicle's velocity can be obtained with high accuracy. Compared with traditional laser velocimeters, the center working distance and the depth of field are increased by this portable laser Doppler velocity measuring system. The center working distance of this novel laser Doppler velocimeter is 10 m; the depth of field is ±1 m, and the velocity measurement accuracy can reach to 0.1%.