The invention belongs to the field of outer transmitter-based radars, and relates to an outer transmitter-based radar system based on multi-FM broadcasting and a detection method. The outer transmitter-based radar system based on the multi-FM broadcasting comprises receiving antennas connected in sequence, a simulation case containing a signal simulating and receiving module, a CPCI case containing a signal collecting and processing module, at least one disk array case containing a signal storage module, a switch and a signal processing machine carrying out signal processing. A multi-FM broadcasting irradiation source adopted by the system has multi-frequency, multi-station and multi-polarization characteristics, a multi-polarization mode is adopted by the receiving antennas, a radio frequency direct sampling structure is adopted for simulating and receiving, a radio frequency band-pass sampling and digital downconversion scheme is adopted by data collection, a high-speed disk array is adopted for data storing, the signal processing machine completes clutter rejection, mutual fuzzy function operation, peak value detection, constant false alarm rate detection, locating and tracking, data fusion and other operations, and finally object distance, speed and azimuth information is output. The outer transmitter-based radar system based on the multi-FM broadcasting and the detection method have the advantages of being free of electromagnetic pollution, resistant to stealth, and high in detection stability and tracking continuity.