Experimental results of thermal effect in laser diode (LD) three-direction side-pumped Tm,Ho∶ LuLiF laser are reported. It is calculated of the distribution of pump intensity in the laser medium. The effects on the pump intensity and uniformity under different pump waists are analyzed. Temperature distribution in the laser media is simulated by finite element analysis. Considering the laser medium as lenslike, the experiment measuring is developed. Using the intensity changing of Gaussian beam through small aperature, the Gaussian beam shapes of He-Ne laser through the laser medium is fitted. Caculating from the approximate transformation of Gaussian beam by thin lens, the thermal focal length of Tm,Ho∶LuLiF crystal is about -2.3 m with pump pulse power of 3.3 J and pulse repetiton frequency of 10 Hz. The experimental result is in agreement with the theoretical prediction.