Abstract: The vehicle detector in intelligent traffic system is studied and developed by using radar principle to finish the real time collection of traffic flow parameters such as vehicle velocity, flow rate. The Doppler frequency of moving target can be used to detect the moving vehicle, and the range measuring principle of FM radar can be used to detect the still vehicle. The digital signal processor transforms and processes the signal in frequency domain, and gains the vehicle velocity and flow rate rapidly and accurately. The vehicle detectors have been applied in the Beijing intelligent traffic system, the detecting precision of velocity and flow rate is up to 95%.