Expounded the fundamental operating principle and main characteristics/features of the microwave vehicle detector.A city trunk road and an arterial road were selected in the investigation to monitor traffic flow by both equipment and labor monitoring,which provides the traffic flow data needed in traffic noise monitoring for comparison.The comparing result showed that the error rate of the whole traffic flow during day and night/(traffic flow round the clock)is about 5%,which indicated that this instrument was applicable to contiuous monitoring of traffic flow.