利用快速傅里叶变换算法和薄层增益近似方法，在Matlab计算平台上，对大长宽比正支共焦有源谐振腔进行了数值模拟。得到了长宽比为3.6∶1的大功率化学激光器输出的近场模式强度和相位分布，以及理想聚焦下的远场光斑的强度分布。给出了由非均匀增益引起的强度分布不均对远场光斑的影响。得到的近场图像结果和实验结果基本一致，表明本方法可以对大长宽比矩形腔的输出模式进行仿真，从而为激光器的光束质量改善提供计算参考。

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.