A novel method of microstrip line-fed excitation source is presented in this paper to analyze microstrip problems by the finite-difference time-domain method (FDTD). Unlike previous excitation treatment, which can only be used to calculate Gaussian pulse incident wave, this method can be used to calculate Gaussian pulse incident wave and harmonica incident wave. It is not necessary to switch the source wall to absorbing boundary condition in the iterative process. Because of field zone division, reflected fields naturally separated from total fields.