The geothermal high-temperature environment will potentially damage engineering, which is mainly reflected in the expansion cracking caused by delayed ettringite formation (DEF). The solution method was used to simulate the temperature and humidity coupled liquid-phase environment of ettringite formation, investigate the temperature limit of ettringite generation in concrete pore solution, and analyze the effect of gypsum admixture on DEF under a geothermal environment. The results show that the high temperature has a promoting effect on ettringite generation from the early hydration of cement, and the high temperature causes the AFt generated early to be gradually converted into AFm as the curing age increases. 70–75 °C is the key temperature interval for the transformation of the AFt crystalline phase into the AFm crystalline phase, and it is recommended that the core temperature of concrete during hardening be controlled below 60 °C to avoid the potential risk of AFt decomposing into AFm in large quantities and DEF. The sulfate content of the Portland cement used in the high-temperature region should be strictly controlled, and the amount of gypsum used should not exceed 4%.