Shell matrix proteins control the nuleation, size and crystal phase of calcium carbonate during the biomineralization of pearl, and they are related to pearl quality. In order to study more about the molecular mechanism of nacre formation, we cloned a novel shell matrix protein silkmaxin (accession No. MK188932) from freshwater mussel Hyriopsis cumingii. With the gene expression detected by RT-PCR and in situ hybridization, silkmaxin was specially expressed in the pallial epithelial cells of mantle outer fold, which indicated that silkmaxin is a nacreous layer matrix protein. The amino acid sequence of silkmaxin featured high proportion of Gly (33.0%) and Ser (10.4%) residues, and the second structure is mainly composed of β-folds. The high structure prediction indicated that silkmaxin is a silk-like protein. In addition, the expression of silkmaxin was detected in the early stage of pearl formation by real-time quantitative PCR. The results indicated that silkmaxin played important roles in the transition of the calcium carbonate disordered deposition to ordered deposition in the early stage of pearl formation. The study of shell by silkmaxin RNAi treatment indicates that silkmaxin gene is essential for the correct growth of aragonite tablets, including the size and shape of the newly formed calcium carbonate crystals.