According to the basic principles of the finite difference time domain (FDTD) method, firstly the physical field characteristics and the numerical calculation of the plasma were analyzed; and then the reentry plasma sheath formed and plasma relative dielectric constant of the wave attenuation were introduced; lastly the propagation of electromagnetic waves in the plasma with three-dimensional FDTD method and two-dimensional FDTD method were simulated. Simulation results proved that the FDTD method is an effective method to solve the electromagnetic wave propagation in the complex medium.