Objective To investigate the effect of calcitriol on osteogenic differentiation of mesenchymal stem cells (MSCs) induced by bone morphogenetic protein 9 (BMP9). Methods The experiment was divided into four groups: control group, calcitriol group, BMP9 group and BMP9+calcitriol group. Quantitative PNPP method was used to detect alkaline phosphatase (ALP) activity. RT-PCR and Western blotting method analyzed expression of osteocalcin（OCN）and osteopontin (OPN). Alizarin red staining assessed the formation of mineralized nodules. In addition, the changes of cell morphology and elastic modulus during osteogenic differentiation were studied by atomic force microscope. ResultsCompared with control group, calcitriol alone had no significant effect on the osteogenic differentiation of MSCs, but calcitriol could enhanced expression of osteogenic markers and formation of mineralized nodules induced by BMP9. However, neither calcitriol nor BMP9 could affect elastic modulus of cells. The combined treatment of BMP9 and calcitol could enhance phosphorylation of AKT and β-catenin which were both important for osteogenesis. The pretreated PI3K inhibitor could inhibit phosphorylation of AKT and β-catenin as well as ALP activity in BMP9+calcitriol group. In addition, calcitriol did not affect the BMP/Smad signaling pathway induced by BMP9. Conclusions Calcitriol synergies with BMP9 could promote MSCs osteogenesis by activating the PI3K/AKT signaling pathway. The study about effects and mechanisms of different regulatory factors on osteogenic differentiation of MSCs is of great significance for the treatment of osteoporosis and the development of bone tissue engineering.