The result of influence on outcoupled mode by introducing intracavity mirror tilt-perturbation in positive-branch confocal resonator is analyzed. The intracavity mode distribution and its Zernike-aberration coefficient due to the concave or convex mirror tilt disadjustment are calculated by adopting equivalent lens method. At the same time, the experimental study about the relations of intracavity mirror tilt disadjustment and mode aberration has been performed by using Hartmann-Shack (H-S) wavefront sensor method. Aberrations of outcoupled beam mode are obtained by using mode-reconstitution method. The results show that the intracavity perturbation of concave mirror has a greater effect on outcoupled beam-quality compared with that of convex mirror. For the large Fresnel-number resonator, the tilt angle of intracavity mirror has a close linear relationship with extracavity Zernike tilt aberration coefficient. The ratio of Zernike tilt coefficient approaches to the magnification of unstable resonator if equivalent perturbation is applied to concave mirror and convex mirror respectively. Furthermore, astigmatism and defocus aberration are also increased with the augment of tilt aberration of beam mode. So intracavity phase-corrected elements used in unstable resonator should be close to the concave mirror. The wavefront relationship between the He-Ne laser and the actual laser beam has also been compared.