Results of an experimental and theoretical analysis of the proprerties of a LD pumped laser using a plano-concave stable resonator and KTP frequency-doubling crystal material are presented. The influence of thermal lens effect on laser output mode and beam quality has been discussed briefly by using the transfer matrix theory. In this paper, further experiment on the output beam quality of a LD pumped acousto-optic Q-switched laser has been also performed by adopting Hartmann-Shack wavefront sensor and Zernike mode reconstruction theory. Using this method, the PV and RMS values of the aberration wavefront, the frontal 35-order Zernike aberrations can be acquired, and the 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. The experimental results show that the beam wavefront aberration is mainly concentrated in the frontal 15 orders of Zernike aberration, mainly including the defocus A3, the low-order astigmatism A4 and A5, and the coma aberration A6 and A7 because of crystal thermal effect.