Abstract-This paper brings forward a simple local approximation finite-difference time-domain (FDTD) method for the analysis of short apertures with a finite thickness. By applying the equivalence principle together with a simple local approximation, the varying field distribution is accurately derived. The updating equations for the slot field can be derived by casting the field distributions into the contour paths containing the apertures. The method has been applied to two problems and the results are compared with the high-resolution standard FDTD simulation results and the measurement results. The accuracy of the proposed method is verified from the comparison of both the field distribution and the time-domain and frequency-domain slot coupling results. It is demonstrated that the local approximation is highly efficient and timesaving, and the present method is stable, numerically and computationally efficient.