Background: Osthole is extracted from Fructus Cnidii and is proved to be effective in the treatment of osteoporosis in rats. However, data are still scarce and the mechanism remains elusive. Objective: To investigate the effect of Osthole on proliferation and differentiation of osteoblast. Materials and Methods: Cells were divided into five groups: control group, β-estradiol group (10−8 M), and Osthole groups (10−6 M, 10−5 M, and 10−4 M). Osteoblast proliferation was evaluated by Cell Counting Kit-8 (CCK-8) assay. Alkaline phosphatase (ALP) activity was detected by ALP staining and enzymatic measurement. Mineralization was detected by alizarin-red staining. The level of osteocalcin was measured by enzyme-linked immunosorbent assay (ELISA). The expression of key proteins of Wnt/β-catenin signaling pathway and endoplasmic reticulum stress (ERS) was analyzed by Western blot. Results: Cell proliferation was retarded in moderate-dose and high-dose Osthole group at 1d, 2d and 3d and in low-dose Osthole group at 1d and 2d (P < 0.05). ALP activity was enhanced in high-dose Osthole group from 1d to 3d and in moderate-dose Osthole group at 2d (P < 0.05). Mineralization of bone matrix was promoted in high-dose Osthole group at 21d (P < 0.05). The secretion of osteocalcin was promoted in Osthole groups at 21d (P < 0.05). Expression of CHOP, GRP78, PDI, Wnt1, and β-catenin was upregulated in high-dose Osthole group at 2d, indicating that both of ERS and Wnt/β-catenin signaling pathway were activated. Conclusion: It can be concluded that the effect of Osthole on inhibition of proliferation is relevant with activation of ERS, and activation of Wnt/β-catenin signaling pathway is one of the mechanisms how Osthole promotes osteoblast differentiation. In summary, this study provided more evidence for Osthole as a potential anti-osteoporosis medicine.