The mechanism investigation of beam quality evolution within laser oscillator is of crucial importance for high beam quality laser output. Combining the thermal effect and gain guiding effect, a theoretical model of laser oscillator is presented. The beam quality evolution inside a laser oscillator is numerically simulated and experimentally verified. The mechanism of beam quality evolution is investigated based on the theoretical model. When the laser beam with negative spherical aberration passes through the laser crystal, the negative spherical aberration can be compensated by the positive spherical aberration of the crystal, resulting in beam quality improvement. This mechanism is meaningful for high beam quality output of laser oscillator and amplifier.