Many factors of intracavity perturbation degrades the output power and beam quality for high power laser diode pumped solid laser. Firstly,the relation between the focal length of thermal lens and the beam spot size is analyzed by means of transmission matrix optics. Experiment on output beam quality of a LD pumped acousto-optic Q-switched laser has been also performed by adopting Hartmann-Shack wavefront sensor and mode method of wavefront reconstruction theory. Using this method,the peak-to-valey value and root mean square values of the aberration wavefront,the frontal 35-order Zernike aberrations can be acquired,and Strehl ratio curve of the output beam and the distribution of circle energy can also be obtained by calculation,so the mode properities can be fully understood. Experimental results show that the beam wavefront aberration is mainly concentrated in the frontal 15 orders of Zernike aberration,mainly including the defocus Z3,the low-order astigmatism Z4 and Z5,and the spherical aberration Z10 because of crystal thermal effect.