The spherical aberration of the thermal lens of the active media is obvious when the solid state lasers are strongly pumped. It can change the mode profile, the single-pass loss, the output power and so on, especially in a large-volume TEM00-mode resonator. The self-consistent mode is calculated using Fox-Li diffraction iterative algorithm. There are side lobes with the fundamental mode when the spherical aberration is introduced, hence the beam quality is degraded. The calculation results agree well with the experimental results. The beam filling factor plays an important role in the design of a laser system. The optimized value of the beam filling factor should be determined experimentally according to the spherical aberration.