The hydration degree and mechanical property of paste and the polymerization of C S H gel in interfacial transition zone(ITZ) between lightweight aggregate and cement were quantitatively characterized by solid state 29Si magic angle spinning nuclear magnetic resonance(29Si NMR) and microhardness tester. The influence of water absorption ratio of lightweight aggregate on mechanical property of paste and microstructure of C S H gel in ITZ was investigated, and the internal curing effect of pre wetted lightweight aggregate was verified by this method. The results show that the hydration degree and microhardness of paste and the polymerization of C S H gel in ITZ are higher than that of the cement matrix. With increasing water absorption rate of lightweight aggregate, the hydration degree and microhardness of paste and the polymerization of C S H gel in ITZ increase, the internal curing effect of pre wetted lightweight aggregate is improved.