Abstract: A novel equivalent circuit model for field-line coupling analysis inside an enclosure with electrically large apertures was proposed in this paper. Firstly, the electric field inside the enclosure was calculated based on Cohn model and mirror image principle. Secondly, a scalar quantity Green function was deduced for electric dipole radiation field inside an enclosure, an integral equation for field-line coupling was expanded in series, and then a matrix equation was obtained, based on which the equivalent circuit model was established. Finally, the boundary condition of the transmission line was considered to compute the terminal response current, and a simulation analysis was carried out for the response current of the transmission line inside a cavity illuminated by a plane wave (<i<ϕ </i<=0,<i<θ</i<=0,<i<φ</i<=π/2). The results of 5 verification tests shows that:the precision of the equivalent circuit model is inversely proportional to transmission line exponent <i<η</i<, and when <i<η</i<=0.375, the correlation coefficient <i<ρ</i<=0.987 8. The ratio of the consuming time is 1:9.2 for 1 001 samples in a frequency range of 0~3 GHz, verifying the efficiency of equivalent circuit model. The correlation coefficient is up to 0.984 4, while the mean of absolute error and the standard deviation of this method compared to TLM are 12.445 dBA and 16.438 dBA, verifying the accuracy of equivalent circuit model.