The heat generation in Nd:YAG ceramic lasers under the conditions of laser extraction was investigated through the heat generation coefficient, which was defined as the ratio of heat generation power to laser output power. The heat generation coefficient of the 1.0% neodymium-doped YAG ceramic laser was calculated to be 0.63. The main factors which influences the heat generation coefficient were discussed in this paper, and the results showed that the heat generation coefficient was very sensitive to the changes in quantum emission efficiency, overlap efficiency and laser extraction efficiency. This work provides a reference for further research of thermal effect of Nd:YAG ceramic lasers.