A vehicle flow detection system is designed on the base of the pymelectric infrared devices. It consists of a weak signal processing and amplification circuit of pymelectric infrared sensor, real-time clock circuit, STC89C52 microcomputer and its peripheral circuit. The system is placed at an appropriate location of the road side, and when a car passes by the detection area, the system composed by two sets of pymelectric detectors with a certain spacing can well receive successively the infrared radiation emitted by the vehicle because of the good focusing ability of Fresnel lens. The time interval between two sets of infrared signals is calculated by the clock circuit to judge whether the vehicle is passing by. The result will be shown on the 12864 LCD screen. Through the outside tests, the system can realize the on-line inspection of traffic flow, and has a higher ratio of quality to price.