Calcium carbonate (CaCO3) is widely used as an important model system for investigating inorganic precipitation reaction or crystallization. However, recent results show that the yield of vaterite CaCO3 microspheres is poor in ethanol/water in the presence of polyelectrolyte poly(sodium 4-styrenesulfate) (PSS), which is up to 16 mM. We now report on an approach to synthesize pure vaterite CaCO3 microspheres through improving the concentration of polymer PSS and the yield is greatly high up to 80 mM. The exploration provides the possibility for large-scale synthesis of CaCO3 materials with controllable morphology and crystallographic structure in aqueous solution at room temperature. The possible formation mechanism toward the occurrence of vaterite CaCO3 microspheres has also been illustrated in virtue of a series of time-resolved experimental results. It is revealed that the vaterite microspheres evolve gradually from the initial amorphous precursor, to poorly crystallized nanoparticles, to sphere-like aggregates, and then to vaterite microspheres embedded with the calcite rhombohedra, finally to the vaterite microspheres with smooth surface. This research may provide a new viewpoint into the forming process of vaterite CaCO3 microspheres.