Abstract: Objective: To study the alleviating effect of oyster shell powder on glucocorticoid induced osteoporosis in mice. Methods: Female ICR mice were randomly divided into four groups, including control, model, calcium carbonate and oyster shell powder groups. Intramuscular injection of 1 mg/kg/d dexamethasone was used to induce osteoporosis in mice and the femoral microstructure, blood calcium, fecal calcium, bone calcium, bone turnover markers and gut bacteria changes were analyzed. Results: Oyster shell powder treatments improved bone microstructure which was characterized by significantly increased bone mineral density and trabecular thickness(P<0.05) and reduced trabecular bone separation and structural model index. The calcium content in blood was also reduced after oyster shell powder treatments(P<0.001). Oyster shells powder significantly increased the enzyme activity of serum alkaline phosphatase(P<0.001) and reduced the enzyme activity tartrate-resistant acid phosphatase(P<0.01). The gene transcription of the bone formation marker alkaline phosphatase and osteoprotegerin were significantly increased(P<0.05), while the gene transcription of the bone resorption markers tartrate-resistant acid phosphatase and sclerostin were decreased(P<0.05). Oyster shell powder modulated the structure of gut microbiota with increased abundance of Lactobacillus johnsonii and Eubacterium coprostanoligenes. In addition, Lactobacillus acidophilus, Lactobacillus caviae, Lactobacillus reuteri and Bacteroides acidifaciens were negatively correlated with bone formation indexes, while positively correlated with bone resorption indexes. Conclusion: Oyster shell powder inhibited bone mineral loss, increased bone mineral density and trabecular thickness, promoted bone formation indicators and inhibited bone resorption indicators, modulated the gut microbiota and thus effectively alleviated osteoporosis.