Intracavity perturbation will bring degrade of beam quality in high power laser. The result of influence in eigenmode by introducing intracavity tilt perturbation in different place of positive-branch confocal resonator was analyzed. At the same time,idea eigenmode,the mode of intracavity tilt perturbation in different place,Zernike aberration coefficient and the far field of beam were all analyzed by using numerical prony method. The results show that the sensibility is increased from convex mirror to concave mirror for a given aberration perturbation in different intracavity place. High order aberration such as defocus and astigmatism will also rise when tilt aberration increases in intracavity mode. So intracavity phase-corrected element used in unstable resonator should be near to concave mirror.