In this paper,it was addressed a hydrogen absorbing and desorbing thermodynamics in α+β type TC21 titanium alloy with high strength and toughness based on thermodynamic experiments and calculation.The rela-tionship between concentration(C),temperature(T),and pressure(P)of TC21 alloy is shown by P-C-Tcurves during hydrogen absorption and desorption process,which were measured by multistep hydrogenation/dehydrogenation methods from 625 to 750 ℃.The P-C-T isotherms at a given temperature were separated into three regions.The partial thermodynamic functions of hydrogen reaction were evaluated by a modified form of Sievert's law and P-C-Trelation of different regions was expressed by the modified Sievert's law.The results show that the enthalpy of hydro-gen reaction in the first and third region relies on hydrogen content.According to Vant's Hoff law,enthalpy and entropy of hydrogenation platform in TC21 alloys are-53.58 kJ?mol-1 and-127.41 J·K·mol-1,respectively.Compared with P-C-T curves of hydrogen absorption,that of hydrogen desorption exists hysteresis.