Abstract: When using random coupling model(RCM) to predict the electromagnetic effect of complex cavity, it is often achieved by measuring the radiation impedance. However, in the process of experiment, it is demanding to meet the conditions of chaotic cavity, cavity processing of coupling channel, simulation of experiment process and so on, and the experimental steps are cumbersome. In order to overcome the above problems, time gating method(TGM) is used to calculate the radiation scattering parameters of the cavity by frequency domain-time domain-frequency domain conversion of scattering parameters and window selection function. The influence of gating time on the calculation results is analyzed. The applicability of this method is validated by comparing the statistical characteristics of the experimental results and the calculated results of time gating method in different frequency bands. Time gating method combined with the random coupling model can simplify the cumbersome process of the original random coupling model for the study of electromagnetic pulse coupling effects in complex metal shielding cavities.