To enhance a missile system's capability of anti-electromagnetic pulses (EMP), the finite integration technique (FIT) was employed to compute the electromagnetic penetration through wing-slot into the missile cavity. The studies of EMP coupling, due to various polarizations and incident directions, slots of varying configurations, with or without the presence of wings were conducted. The results show that the slot has polarization character; The reflection of wings has great impact on coupling coefficients; It is much easier for circularly polarized wave to penetrate into the cavity than other polarized waves; The resonance frequency is mainly influenced by the length of slot.