Abstract: To measure the energy within the transmission line of the high power microwave source, liquid absorption energy calorimeter with full absorption is developed. The mixture of ethanol and deionized water is used as absorbing liquid, the liquid level of the detector rises when the microwave energy is absorbed, and it is detected by the control system as the voltage variation, thus the microwave energy is accurately measured. The structure of the energy calorimeter is simple and has high accuracy. Measurements show that the reflection coefficient of the energy calorimeter is less than -20 dB, the range of the energy calorimeter is 10-600 J, the standard deviation of the five measurements is less than 5%. It can meet the requirements of HPM sources within the transmission line or space radiation energy measurements.