The experiments of wet packing density measurement and flow spread test of cement paste with different water solid ratios and different superplasticizer contents were conducted. The variation process of filling water， flocculating water and surplus water in the cement mix with the increase of admixture was analyzed， and the release effect of flocculating water and densification effect of particle accumulation of superplasticizer were quantitatively clarified. A modified water film thickness model considering the effect of superplasticizer was proposed and compared with the existing test results. The results show that the content of flocculating water can be calculated by the difference of saturated water ratio under different superplasticizer contents， which is linearly related to the superplasticizer contents. The modified water film thickness has a good linear correlation with the cement paste expansion， and the model can well predict the working performance of cement paste， which can provide references for scientific concrete mix ratio design.