In this letter, we propose a method for the numerical calculations of the femtosecond laser pulse passed through a subwavelength aperture. The time-dependent laser pulse is decomposed into a series of monochromatic simple harmonic waves.For the light field of the harmonic wave with a single frequency, the numerical calculation is made based on the solutionof the Green's integral equation set of the electromagnetic waves. Such numerical solution is iterated for all the waves withdifferent frequencies, and all the numerical solutions are transformed into the light fields in the time domain by inverse Fourier transform. The light intensity distributions transmitted the subwavelength aperture are calculated and the results show the propagation of the light field is along the direction of the medium interface.