The energy measurement result of CW highenergy laser, acquired by the calorimetric method and traditional data processing method, is influenced greatly by the heat loss as the laser pulse duration can be up to 30s. The effect of heat loss (including heat radiation, thermal conduction and heat transfer) on energy measurement result of CW high energy laser is studied in this paper. An experiment setup and the mathematical model of timetemperature relation were built up,and the correctness of the mathematical model were verified by experiments. The temperature rising curve of the taper energy absorbing cavity according to the relative expression with a corresponding data processing model has been gained. Based on the mathematical model, the measurement result is fitted with the least square method and the fitted curve tallies with the practical curve quite well. The measurement result was corrected by the data processing model. The energy measurement result shows that the measurement uncertainty is within 1%.