A finite difference method based on Helmholtz equation is presented for the mode calculation of active unstable resonator. The finite difference method for the optical field transmission is analyzed theoretically, and the numerical calculation model for active unstable resonator is established. The optical field distribution of laser resonator output can be obtained accurately and quickly based on the calculation model. The numerical sim. ulation results for bare cavity mode and active cavity mode with non-uniform gain are presented respectively. The simulation results for bare cavity mode are also compared with the modes calculated by fast Fourier transform, and it shows that the numerical simulation methods and the calculated modes are reliable, time-saving, and can be used for model calculation with large Fresnel number.