The effect of aberrations of fundamental wave on second harmonic generation especially on the phase of second harmonic (SH) wave is studied. The solution is applied to type Ⅰ second harmonic generation in KDP crystal. The results of intensity distribution and phase distortion in second harmonic generation are presented when the fundamental wave are Gaussian beams with aberrations of Zernike polynomials,and the effects of different aberration,walkoff and diffraction are analyzed. It is shown that different aberrations have different effects, which is mainly determined by the phase mismatching on the whole aperture; walk off effects bring large tilt and the more serious the effect the more distorted the phase of SH is; and effect of diffraction is neglectable. The approximative formula of SH phase distortion are given when both walkoff and diffration effects can be ignored.