Heat bridge in the online energy measurement device for long-pulse laser enhances heat conduction effect. The research was done to assure the value of the effect on measurement result so as to correct the measurement result. Curves of temperature was divided into three parts,each part was analyzed with Fourier heat law,heat conduction equations and energy conservation law. Relationship between energy loss and device parameter was obtained,and experiment research was done. Experiment data was fitted and conclusion was drawn:heat loss rate is decided by pulse length,heat conduction coefficient of material,length of heat bridge,quality of heat absorption body,and et al.. These parameters are decided according to experiment interval and precision requirement. Theory value and experiment result have a good coherence. Its deviation is 2.9%.