Low-heat Portland cement has the characteristic of stable strength growth at high temperature. In this paper, the strength development of mortar and phase composition, pore development and micromorphology of cement paste under thermal curing (50~80 ℃) after forming and thermal curing after standard curing for 1 d were studied by comparing Portland cement and low-heat Portland cement. The results show that the cement strength collapses under high temperature conditions due to the microstructure deterioration in the later stage of hydration, but this behavior is closely related to the heat in the early stage of hydration. The slow hydration rate of low-heat Portland cement at high temperature makes the hydration products uniform and dense, and the pore structure of paste does not deteriorate with the increase of temperature and the change of heating way. Therefore, its strength can maintain a steady increase at high temperature. The decomposition of ettringite caused by high temperature in the later stage of Portland cement does not directly lead to strength collapse, but the quickly hydration rate of Portland cement in the early stage leads to more holes and defects in the cement paste, accelerates the precipitation and growth of AFm and Ca(OH)2 caused by high temperature in the later stage, and induces the increase of porosity.