The big thermal refractive index coefficient and the thermal expansion coefficient make the thermal effect of liquid laser media greater than those of the solid laser media. Therefore,it is important to study thermal effects of liquid media in gain area during pumping time. According to the energy equation,the wavefront disfortion of the beacon beam induced by the temperature gradient is calculated theoretically in the longitudinal pumping geometry. When the pumping power is increased from 5.9 W to 12.4 W,the Strehl ratio of beacon beam drops from 0.9133 to 0.6174. With the application of wave-front curvature sensing theory,the thermal effect of Rh6G is studied experimentally. Under the same pumping condition with those of theoretical calculation,the Strehl ratio of beacon beam drops from 0.9133 to 0.6174. The beam quality becomes worse seriously.