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This work aimed to use oyster shell powder (OSP) as the partial replacement of Ag/TiO2 particle to obtain multifunctional β-hemihydrate gypsum. Thus, the β-hemihydrate gypsum was mixed with different contents of OSP and Ag/TiO2 particle. Antibacterial and MB removal experiments were conducted to assess the antibacterial characteristic and photocatalytic activity of β-hemihydrate gypsum with Ag/TiO2 particle and OSP. Besides, the formaldehyde degradation test was carried out to evaluate its formaldehyde removal ratio. Moreover, their setting times, compressive and flexural strengths at 1, 3, and 28 days were comparatively analyzed. The experimental results prove that the composite use of OSP and Ag/TiO2 particle provide feasible multifunction for the β-hemihydrate gypsum. They can further improve the bactericidal rates and exhibit extra MB removal ratios compared with the gypsum plasters with single Ag/TiO2 particle. Besides, they can increase the formaldehyde degradation ratios, and this promotion is related to the introduction of Ag/TiO2 particle. However, OSP delays the initial setting time but promotes the final setting time of β-hemihydrate gypsum, and Ag/TiO2 particle hardly affects the setting times. Furthermore, OSP reduces the strengths of plasters at 1, 3, and 28 days. But in general, the composite addition of OSP and Ag/TiO2 particle increase the compressive and flexural strengths of gypsum plasters at 1, 3, and 28 days. These results provide theoretical guidance for the recycling of OSP and the preparation of gypsum-based products with antibacterial and formaldehyde degradation capabilities.