The invention provides a frequency domain time division detection system and method for an external electromagnetic environment. The system comprises an AD sampling module for sampling an echo sequence at a received wave gate; and a DSP signal processing module for obtaining and processing an original echo sample sequence, until a frequency which can cover the energy weakest point of radar frequency domain information of a radar working bandwidth is obtained. The method comprises the steps that the AD sampling module samples the echo sequence at the received wave gate; and the DSP signal processing module for obtaining and processing the original echo sample sequence, until the frequency which can cover the energy weakest point of the radar frequency domain information of the radar workingbandwidth is obtained. The system and the method have the advantages that realization is simple, the system and the method are mainly applied to a narrowband active tracking radar system, an active radar external electromagnetic environment can be detected without depending on a broadband passive receiver, a frequency point with the weakest interference is obtained, cost, volume and weight are superior to those of a conventional method, and the system and method are applicable to engineering application.