Abstract: To further improve efficiency of numerical computation, digital signal processing (DSP) technique is combined with finite-difference time-domain method in computational electrodynamics in this paper. According to concept of signal and systems, computational region is regarded as a linear system. From the symmetric form of Maxwell's equations in active region, the discrete time-domain differential equations have been derived. Then, the linear system matrix form is put forward in the region for solving electromagnetic field. The system diagram of solving differential equations is analyzed in time domain iteration, and corresponding matrix form is given to satisfy the requirement of the DSP. To solve unconditionally stable discrete time-domain equations of electromagnetic fields, alternate-iteration method is used, and differential operator in space region has been decomposed. The system matrix form of unconditionally stable equations is proposed and fulfilled simulation of electromagnetic propagation. Finally, algorithm above has been verified by three examples; such as one-dimensional Gaussian pulse propagation, lowpass filter, and compact band-elimination filter. Results show the proposed method available.