The transient temperature distribution of 18 mm×18 mm×10 mm slate Nd:GGG gain medium of the solid-state heat capacity laser pumped by a 104 W laser diode array is numerically calculated to analyze the influence of the large scale temperature variation on the specific heat of Nd:GGG crystal. Through comparing the calculated results with the experimental data, which is achieved with the HR-2 thermal infrared imager from the end face and the side face of the Nd:GGG crystal, valuable conclusions are obtained.