Abstract: High-temperature characteristics of CaO grain cluster system were studied by using molecular dynamics (ReaxFF-MD) simulation method of the ReaxFF reaction force field. Influence mechanisms of temperature and H₂O and CO₂ atmospheres on the CaO grain agglomeration process were explored, and sintering mechanism of CaO was revealed by tracking the evolution of grain boundary structure and chemical bond changes. Results show that at high temperatures, CaO microcrystals form structurally ordered polycrystals through crystal surface contact, and then form clusters through "contact neck". The contact neck gradually grows under the action of material flow migration, and the clusters are sintered into structurally disordered nanoparticles. Increasing the temperature can accelerate the migration of CaO cells and atoms, promote the integration of the cells, change the grain accumulation state and the clusters tend to be spherical, and intensify the sintering. The atmosphere of H₂O and CO₂ has little influence on the accumulation state of CaO cells after agglomeration and sintering. Compared with CO₂ atmosphere, H₂O has little influence on the agglomeration rate of CaO cells. Temperature is the key factor determining the agglomeration and sintering of CaO microcrystals. When H₂O, CO₂ and CaO microcrystalline clusters come into contact, chemical reactions are highly likely to occur. And the hydrogen ions generated by the dissociation of H₂O continuously combine with lattice oxygen and diffuse into the interior of the clusters, affecting the structure of the cluster and resisting agglomeration. While CO₂ is adsorbed on the surface of CaO, reducing the surface energy. At the same time, its products hinder the bonding of CaO microcrystals and slow down the sintering process.