Objective: To investigate the effect of Inorganic polyphosphate (Poly P) on the proliferation and energy metabolism of tendon stem cells. Methods: Rat tendon stem cells were isolated and cultured, with the stem cell surface markers confirmed by flow cytometry analysis. These rat tendon stem cells were treated with different concentrations of Poly P (0, 0.5 mM, 1.0 mM), and their proliferation was tested by CCK8 assay and the cell cycle analysis. Also, the energy metabolism of these tendon stem cells under the treatment of polyphosphate was assessed by studying the levels of ATP production, and the expression of Collagen I was detected by Western blot and immunofluorescence. Results: The successful isolation of the rat tendon stem cells was confirmed by the flow cytometry analysis. We found that Poly P could dose-dependently promote the proliferation of tendon stem cells and increase the proportion of cells in the G2/S phase. Immunofluorescence and Western blot both showed that Poly P could increase the expression of Collagen I in these tendon stem cells. Conclusion: Poly P treatment can promote the proliferation of rat tendon stem cells in vitro. It can also up-regulate the expression of collagen I and increase the ATP production of these tendon stem cells in a dose-dependent manner. Understanding the effect of Poly P on the key features of tendon stem cells may provide valuable information for the treatment of tendinopathy.