In the field of artillery and automobile engine ignition， microwave ignition devices have emerged as a prominent area of research. Among these， microwave resonator ignition devices are considered fundamental ignition devices. We propose a fast time-domain finite difference method for simulating the resonant cavity， building upon the traditional finite difference time domain （FDTD） method. The new method writes the differential form of Maxwell's curl equation into matrix form containing the information on the resonant cavity structures and the incident wave frequencies. The instantaneous value of the electric field can be quickly calculated by solving for the power of the matrix. Compared with traditional FDTD， the new method has great time advantages in simulating the resonant cavity’s electric field distribution for a long time. The simulation experiments demonstrate that， for the same resonator cavity， when the simulation time exceeds 100 000-time steps， the proposed method achieves a significant improvement in time efficiency， sometimes even several times or up to 100 times faster. Moreover， the simulation results remain consistent with those obtained from the traditional method.