The influence on an outcoupled mode by introducing intracavity tilt perturbation in an unstable resonator is developed. The intracavity mode properties and Zernike aberration with the intracavity mirror's misadjustment are calculated theoretically. The experimental results about the relations of intracavity perturbation and the properties of mode aberration are presented by adopting a Shack-Hartmann (H-S) wavefront sensor. The results show that the intracavity perturbation of the concave mirror has a more remarkable effect on outcoupled beam quality than that of the convex mirror. For a large Fresnel-number resonator, the tilt angle of the intracavity mirror has a near-linear relationship with the extracavity Zernike tilt coefficient. The ratio of the tilt aberration coefficient approaches the magnification of the unstable resonator if equivalent perturbation is applied to the concave and convex mirrors, respectively. Furthermore, astigmatism and defocus also increase with tilt aberration of the mode. So an intracavity phase-corrected element used in an unstable resonator will be most effective when placed as near as possible to the concave mirror plane. Based on these results, a control system for correcting intracavity tilt aberrations is established, and the aberration-corrected results are presented. (c) 2006 Society of Photo-Optical Instrumentation Engineers.