At the initial stage of cement hydration under geothermal environment, the monosulfoaluminate (AFm) and sulfate are easy to form delayed ettringite (DEF)in the pores or interfaces, which will lead to expansion and cracking of concrete. The solution method was used to simulate the temperature humidity coupled liquid environment of ettringite (AFt) formation, the molecular dynamics simulation method was used to study the internal structure change of AFt under normal temperature and geothermal environment, and the formation temperature limit of DEF and the critical gypsum content in cement were determined. The results show that the geothermal high temperature environment will affect the growth of AFt crystals on (100) and (110) crystal planes, leading to the gradual decrease of AFt formation with the increase of temperature. 70~75 ℃ is the key temperature range for the transformation of AFt to AFm, but AFt can still be formed in the liquid phase environment above 75 ℃, and transformed AFm content will gradually increase with the increase of temperature. High temperature can promote the formation of AFt hydrated at early stage of cement. With the increase of curing age, high temperature will cause that AFt generated at early stage to gradually transform into AFm. Forming amout of AFt in concrete increases with the increase of gypsum content, and gypsum with a mass fraction of about 4% is the appropriate content. The molecular dynamics simulation results also show that AFt structure is stable at room temperature, but its structure changes under geothermal conditions.