It is believed that chitosan plays an important role on the formation of specific inorganic–organic hybrid structure of shells. In this paper, insoluble chitoan was modified to a water-soluble macromolecule-carboxymethyl chitosan (CMCS), and further used as an additive in the crystallization process of calcium carbonate (CaCO3). The combination investigation of XRD, Micro FT-IR and SEM on the precipitated crystals shows that the addition of CMCS obviously changed the crystal morphology, but did not induce the transformation of the crystal form, i.e., CaCO3 keeps calcite form in all the cases no matter whether CMCS exists or not in the precipitation systems. The morphological variation of CaCO3 strongly depends on the CMCS concentration in the aqueous solutions.