Abstract: The resonator’s cross section of high power chemical laser is commonly rectangle with large aspect ratio. Fast Fourier transform (FFT)method and thin gain sheet approximate calculation are used in simulating the optical resonators. Simulations give near field normalized intensity distribution, near field relative phase distribution and far field normalized intensity distribution. The output beam quality influenced by heterogeneous gain was analyzed. The numerical results also illustrated that the normalized central intensity of far field spot will turn dark along with the increasing of geometrical magnification. The method and results provide a theoretical basis for improving the beam quality.