In nature, shells exhibit remarkable high toughness and impact resistance to the external load despite their brittle main constituent and simple hierarchical structure. In this work, the structure of the mussel shell Hyriopsis cumingii is analyzed by scanning electron micro-scope and atomic force microscope, and the macro/micro compression and impact tests are performed. Results show that the shell has a three-layer structure: an outer cuticle layer, a prismatic layer, and a nacreous layer. The stiffer and load-dependent prismatic layer is conducive to improve the impact resistance of shell structure. Fracture morphology after failure proves that cracks are transgranularly propagated inside the prism and aragonite platelet, and the crack deflection and platelet pullout can effectively lock the stress, thereby eventually improving the impact-resistance and toughness of the shell.