Based on the objective fact that the effect of thermal lens and the influence of spherical aberration coexist, the intrinsic theoretical relationship between the beam quality factor M2，the thermal focal length and spherical aberration is established, and the beam quality change under the combined effect of the thermal focal length and the spherical aberration is more accurately analyzed in addition. In this experiment, the Hartmann-Shack wavefront sensor is used to reconstruct the related wavefront information of the Nd∶YVO4 crystal by LD end-pumped. The experimental data of the beam quality factor M2 are compared with the theoretical analysis. Furthermore, under the influence of the thermal focal length and the spherical aberration, the gradual attenuation of the beam quality is proved. The feasibility of this model is finally confirmed. The above conclusions provide the reference for the optimal design and experiment of solid-state laser.