The invention discloses a traffic flow detecting system based on a PSD (position-sensitive detector). A PSD distance measuring module arranged on an upper end support frame of a highway is used for measuring the distance, a change curve of the measured distance of the PSD is detected according to the system, the traffic flow is judged, distance shortening indicates that vehicles pass, in addition, the vehicle types are determined according to the distance change amplitude, through the time difference of the vehicles passing through the front and back PSD distance measuring modules, the vehicle directions are distinguished, in addition, the vehicle speeds are calculated, and finally, the measured traffic flow, the vehicle types, the vehicle flow directions and the vehicle speeds are transmitted to a traffic flow management platform in a wireless way. The traffic flow detecting system has the advantages that the assembly is simple, the operation is easy, and high popularization and application values and wide market prospects are realized.