This study investigated the frost-induced deterioration of C-S-H gel and pore structure in young cement paste by TGA, FTIR, ²⁹Si-NMR, MIP, and nanoindentation, and analyzed the changes in C-S-H gel and pore structure of frost-damaged paste. For C-S-H gel, frost attack causes the loss of C-S-H gel to be as high as 36.3%, decreases the formation of LD C-S-H, and inhibits the transformation from LD C-S-H to HD C-S-H. It also decreases the mean chain length (MCL) and the degree of polymerization (n_c) of C-S-H gel due to the breakage of Q² silicate chain. Even after re-curing at 20 °C, the mass loss from C-S-H gel still exceeds 15%, and C-S-H gel also has lower MCL and n_c values. For pore structure, after suffering from frost attack, pore size distribution curve significantly moves toward the direction of increasing pore diameter. Gel pore and mesopore are decreased due to the packing of large amounts of unhydrated cement particles and few C-S-H gels formed, while capillary pore and macropore are increased owing to the freezing-induced expansion and ice crystal growth. After re-curing at 20 °C, gel pore, mesopore, and macropore approach the unfrozen level, but capillary pore is still higher than the unfrozen level.