To analyze the electromagnetic environmental effects of composite shell platforms in strong electromagnetic environments such as nuclear electromagnetic pulses, we have obtained an equivalent calculation method of the finite-difference time-domain method in dealing with weakly conducting thin-layer dielectric materials based on the integral form of the Maxwell-Amper theorem. The thin layer model can be appropriately thickened while proportionally reducing its conductivity when the equivalent wavelength of the medium is larger than the model thickness. The electromagnetic coupling characteristics of the model before and after parameter equivalence are essentially the same. This method can reduce the computational effort by increasing the grid step size. In addition, this method does not require changing the time step format of the traditional finite-difference time-domain method and does not affect the stability of the calculation. Numerical experiments, such as the examples using infinitely large thin plates, thin spherical layers, and electromagnetic coupling of unmanned aerial vehicles with thin shells, have shown that it has good applicability to the electromagnetic coupling simulation of thin-shell platforms containing weakly conducting materials with millimeter thickness in nuclear electromagnetic pulse environments.