The absolute volume change of cement paste in cement hydration process was measured by helium pycnometry. Based on the absolute volume change curve of cement paste, hydration process of Portland cement can be divided into 4 stages, i.e. flash shrinkage stage, shrinkage suspension stage, fast shrinkage stage and shrinkage slowdown stage. Obvious absolute volume change of cement paste occurred in flash shrinkage stage and fast shrinkage stage, and the change ranges were huge, due to the hydration of C3A and C3S. The absolute volume change of cement paste kept decreasing linearly with a very slow rate in shrinkage suspension stage, and its mechanism seems to fit the saturation theory. Differently, there came up “inflation” illusion on the absolute volume change curve of cement paste in shrinkage slowdown stage, and the curve can not reflect the real absolute volume change of cement paste any more. However, this phenomenon was closely related to the development of the pore structure of cement paste, which was important for the characterization of cement hydration behavior. Water cement ratio can affect not only the hydration process of Portland cement, but also the absolute volume change of cement paste in each hydration stage.