In this paper, a typical backdoor coupled target is constructed, and the echo characteristics of the target are simulated from two dimensions of time domain and frequency domain. It is found that amplitude pits can be observed in the echo frequency domain waveform when the aperture-cavity structure of the target is strongly coupled, and the echo time domain waveform when the aperture-cavity structure is strongly coupled is bimodal, which is obviously different from the non-strongly coupled echo. By changing the size and shape of the backdoor coupled target, the characteristic law of the echo signal is verified by simulation.