Water in the nanometer to micrometer-sized pores of calcium silicate hydrate (C-S-H) is essential for the binding process of cementitious materials. The quantity, location, and physical state of water in C-S-H pores under extreme conditions significantly influence the strength and durability of cementitious materials. The present study employed ReaxFF and molecular dynamics (MD) simulation to evaluate the effects of water ultra-confined in the nanopores on the structure, bonds, dynamics, and tensile mechanism of the C-S-H grains at elevated temperatures. The results indicate that the temperature elevation may interfere with the water molecule's hydrogen-bond network between the C-S-H grains, causing a notable nanometer-scale pore expansion. Simultaneously, the diffusion coefficient of water molecules confined in nanopores gradually increased, and their dynamic characteristics shifted from a glassy nature to free water. Additionally, high temperatures promoted hydrolysis reactions and the breakage of chemical bonds in the C-S-H framework, causing disintegration of the silicate skeleton and a decrease in the mechanical attributes of C-S-H. Moreover, the uniaxial tensile test at high temperatures revealed that the silicate chain groups in the C-S-H substrates underwent thermal curling. In contrast to interlayer-bound water, under the action of tension, water molecules in nanopores are viscous, forming water layers.