Abstract: The calibration of energy calorimeter is one of the important aspects in the measurement of the energy of high power laser facility. Using the method of linearity fitting is suggested in the calibration of energy calorimeter. The standard deviation of slope is used as the main criterion for deciding which linearity fitting is adopted, and the correlation, the intercept, and the standard deviation of intercept are taken into account. If the slope standard error of zero crossing linear fitting is less than that of un-zero crossing linear fitting, then the intercept is considered as a random error. The calorimeter coefficient is solved according to the method of weighted average of un-equal precision. Otherwise, a great system error exists, the calorimeter coefficient is obtained by un-zero crossing linear fitting, and the value of intercept is a system error. According to the value of regression error, we can determine whether the gross error exists and how to eliminate it. In this way, the appropriate calorimeter coefficient is obtained. The analyzed results are of reference value for the exact measurement of laser energy.