Thermal effects and beam wavefront properties of a diode-pumped Q-switch Nd:YAG laser have been investigated. The influence of thermal lens effect on laser output mode and beam quality is discussed by means of transmission matrix optics. The functional relations between the thermal lens focal length and the beam spot size for a plano-concave stable resonator have been calculated numerically. Further experiment on the output beam quality of a laser diode pumped acousto-optic Q-switched laser has been also performed by using a Hartmann-Shack (H-S) wavefront sensor and Zernike mode reconstruction theory. In this way, the PV and RMS values of the wavefront aberration, the first 35 orders Zernike aberrations, can be acquired, and point spread function (PSF) of the beam and the distribution of circle energy can also be obtained by calculation, so the output beam quality can be known fully. The experimental results show that the wavefront aberration of laser is mainly includes the defocus Z3, the low-order astigmatism Z4 and Z5, the spherical aberration Z10, etc. because of the thermal effect of gain medium, which will lay an experimental foundation for solid-state laser optimum design.