To accurately analyze the shrinkage behavior and mechanism of cement-based materials containing calcium sulfoaluminate expansive agent under drying conditions and reveal the mechanism of the decline in the compensatory shrinkage effect of calcium sulfoaluminate expansive agent under low relative humidity conditions, the non-destructive low-field nuclear magnetic resonance relaxation technique was adopted to monitor the development of drying shrinkage of white cement paste with and without calcium sulfoaluminate expansive agent at different levels of relative humidity for 10 months, as well as X-ray diffraction test, mass and length monitoring for specimens. From the perspective of both water content and its state, the evolution of drying shrinkage of white cement paste was quantitatively described, the relationship between drying shrinkage and relative water loss was established, and the mechanism of CSA increasing drying shrinkage was elucidated. Results indicate that, dry in environments with 75%, 43% and 11% RH, water loss and drying shrinkage of white cement paste continually increase until stabilization. The relationship between drying shrinkage and relative water loss of specimens in the three types of environments remained almost the same.With the extension of drying time, the drying shrinkage caused by unit water loss initially decreases and then increases. This change can be explained by the difference of water loss in pore at all levels in cement paste during the drying process, and the non-identical mechanism of drying shrinkage under different humidity conditions. When calcium sulfoaluminate expansive agent is mixed into cement-based materials, C-S-H gel nanopore structure will be coarsened to some extent, easy to lose more evaporable water when drying, and ettringite generated by the hydration of the expansion agent will also lose 2 to 5 water of crystallization, both of which together lead to the net slurry moisture loss and drying contraction are increased. In the application of calcium sulfoaluminate expansive agent in the low humidity environment, it needs to pay attention to these two factors leading to the contraction of the impact of increased fall.