Taking account of the fact that the laser medium always cannot contact completely with the heat sink, the temperature of the laser medium is computed and its thermal effect is studied. Phase difference near the surface has a sizable fluctuation and therefore complicates the thermal effect. The fluctuation near the surface becomes greater with the increasing pump power, while the change of its depth is unconspicuous. The phase fluctuation as well as its depth reduces with the increasing contact area. At low pump power, the diffraction loss increases linearly with it, the larger the beam radius, the bigger the slope, when the pump power increases to some values, the slope becomes smaller, especially the beam with larger radius. Compared to the smaller radius beams, the larger radius beams has bigger losses for all powers. The variation of the diffraction loss with the contact area is not significant in small beam radius, while the diffraction loss increases with the contact area reduces in larger beam radius.