For the conventional unstable resonator, the output laser is deteriorated by the hard-edge diaphragm diffraction. Using variable reflective mirror, as a soft-edge diaphragm, the intensity distributions of output laser are improved. Gaussian mirror has been used widely, and the super-Gaussian mirror is a development for variable reflective mirror. The fast Fourier-transform (FFT) algorithm is a convenient method for simulating the intensity distribution of resonator. In this paper, the three-dimensional intracavity distribution of super-Gaussian mirror resonator is analyzed, and the intensity distributions are simulated when the thermal focal length changes. Then the output intensity distributions are simulated with or without considering the thermal lens effect. And the influence of the thermal lens effect is discussed.