In order to explore the action mechanism of high polymer (HP-SRA) by comparison, the self-made high polymer (HP-SRA) and Eclipse-Floor shrinkage reducing admixture (Eclipse-SRA) were selected as two shrinkage-reducing materials. The results indicate that the shrinkage reducing ability is improved through reducing the average adsorption rate of high polymer (HP-SRA) on the surface of cement particles, the surface tension and the evaporation of pore solution, and through improving the kinematic viscosity of pore solution and the distribution of capillary structure of cement. Meanwhile, The dispersion ability is improved through improving the initial fluidity cement paste, and the fluidity had nearly no loss after 1 h. The unity of shrinkage-reducing and dispersing performance is achieved.