In order to explore the effect of unstable resonator stability on laser beam quality, the numerical simulation of midinfrared laser and visible laser was carried out in GLAD software. The simulation results showed that the existence of defocus aberration, tilt aberration and astigmatic aberration in the unstable resonator can cause the center of the far-field spot of the output annular beam to drift, the number of peripheral diffraction rings to increase, the beam quality to deteriorate, and the degree of effect is different. It is also found that on the basis of the effect of tilt aberration and astigmatism aberration, the introduction of defocus aberration can improve the output laser beam quality to a certain extent. In addition, under the condition of the same aberrations, the effects of different wavelength lasers are roughly the same. However, in terms of the degree of effects, the short-wave laser is much higher than the medium-long-wave laser, which verifies that the resonator debugging of the short-wave laser is more difficult than that of the medium-long-wave laser in the experimental process. The simulation results can provide an important reference for the optimization design of the laser system, the processing of cavity mirror and the formulation of the correction range index of the adaptive optical system.