Abstract: With the illumination of the long pulse laser, the calorimetric laser energy obtained by the traditional data processing differs significantly from the actual laser energy due to the thermal loss. To solve this problem, the mathematic model of temperature distribution of the energy probe under laser illumination was established. Taking heat conduction and radiation into consideration, a measurement method of the thermal loss coefficient for characterizing the extent of thermal loss of the laser energy meter is proposed. The validity of the measurement method was verified by experiments. The obtained thermal loss coefficient can also be taken as an important parameter to evaluate the calorimetric laser energy meter.