Laser optical system aberrations can degrade beam quality, cause deviations from the optical axis during transmission, and shift focus positions, thereby adversely impacting laser processing. In this paper, the influence of aberration on beam quality of resonator with different structures is analyzed theoretically, compared with the negative-branch resonator, the positive-branch resonator is less affected, however, when the equivalent Fresnel number is large, the beam quality of both types of resonators deteriorates sharply. When these two resonators are numerically simulated, it is found that the effect of aberration is small for a positive-branch resonator with an equivalent Fresnel number of 72.6, and experimental results also prove this phenomenon. While a negative-branch cavity with the same gain region volume and equivalent Fresnel number of -223.8 has also been investigated, the output beam direction deviates -1.79 mrad from the optical axis when resonator with spherical mirrors, while the output beam is almost a collimated one and propagates along the optical axis for parabolic mirror resonator because the influence of aberration is eliminated. Therefore, the influence of aberration must be considered in the design of unstable resonator with large Fresnel numbers.