This paper applies an unconditionally stable semi-discrete (SD) scheme to compute the two-dimensional electromagnetic field in time domain. Numerical dispersion of this scheme is derived and compared with the alternate-direction-implicit (ADI) FDTD and the Crank-Nicolson (CN) FDTD methods. The dispersion curve of the proposed scheme is found to be the lower and the upper limits of those of the explicit and the implicit FDTD methods, respectively. As a numerical example, the adaptive Runge-Kutta method is adopted to solve the semi-discrete Maxwell equations for the fields in a 2D TM PEC cavity. Numerical results reveal that the SD scheme is much accurate than the ADI FDTD method. The computation speed, however, still has to be improved.