Using angular spectrum propagation theory and thin gain approximation method, for the gain medium with different small signal gain coefficient, the positive branch confocal unstable resonator optical field mode is calculated on MATLAB platform. The phase and amplitude distribution of optical fields such as Gaussian type, Gaussian type along X direction and polarizing axis Gaussian type along X direction are obtained. Calculation results show that the distribution of the small signal gain coefficient of the gain medium has a decisive influence on the intensity distribution of the optical field.