Under high temperature conditions, the presence of defects and confined water in C-S-H gel significantly affects its strength and durability. This study employed reactive molecular dynamics to investigate the molecular structural characteristics and reactivity of type I defect C-S-H gel under high temperature environments. The findings reveal that the increase in temperature and Ca/Si ratio enhances the mobility of water molecules, accelerating their penetration into the layered structure, reducing structural order and connectivity, leading to an increase in Q¹ and Q⁰, and weakening the thermal stability of C-S-H gel. Type I defect causes the C-S-H gel to expand along the interlayer direction, and the expansion rate increases with rising temperature and Ca/Si ratio. Under high temperature and high Ca/Si ratio conditions, the reactivity of water molecules is enhanced, promoting hydrolysis reactions and the formation of hydroxyl groups, and type I defect rapidly expands, significantly affecting the structural stability of gel. The results of this study provide theoretical support for improving the performance of concrete in extreme environments such as fires.