In order to improve the vehicle safety performance, a signal processing algorithm for active anti-collision millimeter wave radar based on time frequency analysis and constant false-alarm rate (CFAR) is proposed. Time-frequency analysis method is firstly used to model the mechanism of frequency modulated carrier wave (FMCW) radars in detecting targets. Then, the design of the signals processing flow is detailed. The CA-CFAR algorithm and the improved algorithm (GO-CFAR) are discussed in detail and verified by simulation experiments and field experiments. The results show that the algorithm has high precision in detecting the speed and distance parameters of the target and computing target in the expressway environment.