Abstract: Interlayer performance of layers-poured mass concrete is closely related to the water state of lower fresh concrete layer. This study focuses on variations in water content of such concrete and the mechanism of water migration and offers a guide for pouring construction in layers. We used the low field nuclear magnetic resonance technology and further developed it to measure the water content and its distribution in fresh concrete with different compositions and different water-cement ratios under various pouring environments. The results indicate that the major factors in water migration during concrete setting are hydration, evaporation and migration and that the trends of variations in water content and its spatial distribution are controlled by changes in the pouring environment, especially those changes in the surface layer. A further examination on the patterns and mechanism of water migrating inside the concrete reveals that the migration is dominated by quasi-laminar flows driven by density differences at early setting stage while by diffusion under humidity gradient at late setting stage.