Ocean is a largest carbon reservoir in the Earth, and the total production of marine shellfish in China is huge. Most of shellfish only utilizes their edible value, and shells are randomly stacked or buried as a solid waste. To improve the utilization rate of shellfish waste, the effect of NaAlO2 on the hydration process, solid-liquid phase composition, pore structure and mechanical property of the shell powder-calcium aluminate cementitious system was investigated. The results show that NaAlO2 enables accelerating the hydration process, shortening the setting time, and slightly improving the compactness and mechanical properties of the hardened matrix. NaAlO2 affects the solid-liquid phase composition. Adding NaAlO2 increases the alkalinity of the pore solution and improves the reactivity of CaCO3, leading such a cementitious system to the formation of a stable phase (i.e., monocarboaluminate (C4AcH11)) with a reduced amount of the unstable phase (i.e., calcium aluminate hydrate (CAH10)). The samples with 3.75‰ and 5.00‰ of NaAlO2 produce 13.0% and 13.6% of C4AcH11, respectively, and no CAH10 phase occurs after cured for 28 d. The crystallite size of the microcrystalline AH3 phase grows, which compromises the cementitious property because NaAlO2 increases the alkalinity. The excessive NaAlO2 leads to a decreased mechanical property. Mussel shell-based carbonaluminate cementitions materials hold promise in marine civil engineering construction.