The heat balance and characteristics of steady state circulation flow of a single loop pulsating heat pipe was studied with thermo-mechanical analysis. The relationship of system dissipation work and volume work when the system was running stably and the relationship of water vapor expansion work and saturated pressure with water as working fluid were established. The results showed that dissipation work was necessary to keep stable running，and the heat absorption from the evaporator section must be equal to the heat release from the condenser section. The temperature and pressure of the evaporator section must be higher than those of the condenser section. The water vapor expansion work must be higher than the condenser compression work，and the difference was used to overcome system dissipation work. The graph of water vapor expansion work distribution and its variation with saturated pressure was presented，with a maximum work at pressure 2.5MPa and temperature 225℃.