Abstract: In order to explore the hydration and water distribution characteristics of cement mortar before final setting and reduce the adverse effects of hydration process on the properties of cement-based materials, low-field nuclear magnetic resonance equipment was used to observe the pore size variation and water distribution during the solidification process of cement mortar in real time, and quantitatively characterized the hydration degree and hydration rate of cement mortar based on nuclear magnetic resonance T2 spectrum, which effectively reveals the microscopic pore structure, water distribution and hydration reaction law of cement mortar before final setting.The results showed that the capillary pores gradually changed into transition pores and gel pores of smaller pore size with the increasing of hydration degree.Water transmission and transfer in the internal pores of the specimen were observed.In the first 3 hours, the physical bound water was mainly converted into bled water covering on the surface of the specimen, and some of it was transmitted to the gel pores.After 3 hours, more small pores were generated.Under the action of capillary pressure, the water in the capillary pores was transmitted to the transition pores and gel pores.In addition, the hydration degree gradually increased with the increase of hydration time.While, the hydration rate changed in an inverted S shape, in which it gradually decreased at the initial stage, and remained basically unchanged when it decreased to the dormant stage, and then reacted fast for a period of time and finally to be stable.