The quantitative characterization methods of free water and its action mechanism were studied by relating rheological properties of fresh cement paste to the packing density of cement partials. The results show that free water in cement paste can be classified by the plastic limit and liquid limit. Type 1 free water that is the water exceeding plastic limit can separate cement particles. Type 2 free water that is the water exceeding liquid limit will cause the flow of cement paste under gravity. The improvement of flowability by adding superplasticizer is not only due to the increase of dispersion of cement particles, but also mainly because of lower adsorption force of cement particles to water, lower loose bound water amount of cement particles, and higher free water amount of cement paste. The flowability of cement paste can be accurately and effectively controlled when it is associated with the free water amount.