Background: Nonunion fracture is a serious complication that occurs after fractures, which has significant impacts on the prognosis of patients. Curcumin has been shown to promote bone tissue repair and treat nonunion fractures by regulating the process of osteogenic differentiation. However, the specific mechanism behind this strategy is yet to be explored. This study aims to investigate the potential mechanism of curcumin in regulating the proliferation and osteogenic differentiation of rat bone marrow mesenchymal stem cells (rBMSCs) by modulating the expression of osteogenic-related factors Bone morphogenetic protein 2 (Bmp2), Osteocalcin (Ocn), Osteopontin (Opn), Osterix and Runt-related transcription factor 2 (Runx2) through in vitro cell experiments. Methods: Using rBMSCs for cell experiments, the effects of curcumin on cell proliferation were assessed using the cell counting kit-8 (CCK-8) and cell cloning assays. Different concentrations of curcumin were used to establish reasonable experimental groups. Furthermore, the impact of curcumin on osteogenic differentiation and its regulatory mechanism were assessed. The macroscopic regulation of curcumin on osteogenic differentiation was further investigated by alkaline phosphatase (ALP) staining and activity assay, alizarin red staining, and quantification. Finally, the expression levels of related mRNAs and proteins were detected using Real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot. Results: It was found that compared to the control group (no curcumin treatment), the curcumin 5 μM and 10 μM groups did not have any effect on rBMSCs activity within 48 hours (p > 0.05). However, after 72 hours, a significant increase (p < 0.05) was noted in rBMSCs activity. Furthermore, curcumin 5 μM and 10 μM groups revealed increased proliferation of rBMSCs in vitro (p < 0.05), enhanced alkaline phosphatase activity (p < 0.05), and increased calcium deposition (p < 0.05). They also showed upregulated mRNA expressions of Bmp2, Ocn, Opn, and Runx2 genes and the protein expressions of Bmp2, OCN, OPN, Osterix, Runx2 (p < 0.05). Similarly, the curcumin 10 μM group can upregulate the mRNA expression of Osterix (p < 0.05). On the other hand, group of cells treated with 15 μM curcumin inhibited cell activity rBMSCs after 48 hours (p < 0.05), inhibited the proliferation of rBMSCs in vitro (p < 0.05) and inhibited ALP activity (p < 0.05). However, this group did not affect the deposition of calcium salts, the mRNA expressions of Bmp2, Ocn, Osterix, Opn and Runx2 (p > 0.05), and the protein expressions of Bmp2, OCN, Osterix, OPN (p > 0.05). Additionally, curcumin 15 μM group showed an inhibitory effect on Runx2 protein expression. Conclusions: Curcumin at concentrations of 5 μM and 10 μM can promote the osteogenic differentiation of rBMSCs. Curcumin at a concentration of 15 μM can inhibit the osteoblastic differentiation of rBMSCs.