[Objective] To investigate the effects of inorganic polyphosphate (Poly P) on the proliferation and energy metabolism of ten- don stem cells in vitro. [Methods] Rat tendon stem cells were isolated and cultured, their surface molecules were identified by flow cytome- try, and then treated with 0 mmol/L, 0.5 mmol/L, and 1.0 mmol/L Poly P. The Ckk-8, cell cycle and ATP energy assays were performed. In addition, the immunofluorescence and western blot assays were used to detect the expression of type I collagen. [Results] The flow cytome- try showed that ＞ 95% of the isolated cells expressed CD44 and CD90.1, but almost no CD45 and CD106. In term of Ckk-8 assay, the OD values of in all groups increased significantly over time (P＜0.05) , which increased significantly in Poly P concentration-dependent manner, with statistically significant differences among the three groups from 4 to 24 hours (P＜0.05) . The flow cytometry revealed that Poly P in- creased the percentage of G2+S phase and promoted cell proliferation in dosage-dependent manner with a statistically significant differenc- es (P＜0.05) . In terms of ATP detection, the ATP detection in the three Poly P groups increased significantly over time (P＜0.05) , which sig- nificantly increased in Poly P concentration-dependent manner with statistically significant differences among the three groups from 4 to 24 hours (P＜0.05) . Regarding to type I collagen presentation, the immunofluorescence staining showed that the fluorescence intensity of type I collagen increased significantly with the increase of Poly P concentrations, additionally, Western blot analysis showed that the expression of type I collagen in 0.5 mmol/L and 1.0 mmol/L Poly P group was significantly increased compared with that in 0 mmol/L Poly P group (P＜ 0.05) . [Conclusion] Poly P does promote the proliferation of rat tendon stem cells by up-regulating the expression of type I collagen and ATP production in vitro.