The invention discloses an electromagnetic wave detection method and device based on medium frequency dispersion difference. The method specifically comprises the following steps of: carrying out a surficial electrical parameter test for a target which possibly exists under the ground and the adjacent medium of the target; analyzing the frequency dispersion variation rule of the target and the adjacent medium, and finding a frequency band with the maximum frequency dispersion difference of the target and the adjacent medium to be used as an emission frequency range of an electromagnetic wave; within the emission frequency range, emitting the electromagnetic wave to a detection target, and receiving an echo signal of the detection target; according to electromagnetic wave signals and echo signals of different frequencies, calculating the dielectric constant of the detection target; building a frequency dispersion characteristic curve; realizing target identification by frequency dispersion curve matching; and selecting the echo signal corresponding to the highest frequency within the emission frequency range for inversion imaging to obtain the shape of the detection target. According to the diffidence of different mediums under different electromagnetic wave frequency functions, the physical property information of a medium body is identified so as to achieve a purpose of non-destructive detection.