Abstract: In order to improve the output beam quality and energy for high power lasers,firstly,the influence of intracavity aberrated perturbation on the eigenmode of unstable resonators was analyzed.Then,the effect of intracavity phase-tilted perturbation on the mode intensity and phase distribution for passive positive-branch confocal unstable resonators was calculated by means of numeric iterative method.Finally,the phase property of the output beam was analyzed based on Zernike mode-reconstruction theory so that the front 35-order Zernike aberration coefficients,the point spread function (PSF) and the integrated relative far-field intensity were obtained.Now the laser beam quality was fully illustrated.The results illustrate that in a confocal unstable resonator with a small Fresnel number,small phase-tilted perturbation has not only a remarkable influence on the output beam intensity but also the phase structure,and it makes higher order Zernike aberration such as A3,A7 and A10 increase in the beam phase structure.The results are of some reference to further investigation on the intracavity aberration compensation for high power lasers.