Abstract: Taking ‘Fengdan’ seedlings as the test material, growing in pots and inoculating exogenous AMF Gigaspora rosea and non-inoculating, the plant biomass, chlorophyll relative content, gas exchange parameters, photosynthetic response curve and chlorophyll fluorescence parameters were measured.The effects and response mechanism of arbuscular mycorrhizas fungi（AMF） on photosynthetic characteristics of oil peony Paeonia ostii ‘Feng Dan’ seedlings were analyzed, in order to provide a refernce for improving the growth status and enhancing the yield of ‘Fengdan’.The results showed that the dry weight of aboveground and underground parts and relative content of chlorophyll of the ‘Fengdan’ seedlings that inoculated with exogenous AMF were significantly increased compared to plants that non-inoculated with AMF.Additionally, by inoculated with exogenous AMF,the net photosynthetic rate（Pn） under natural light intensity of plants increased by 37%,indicating that the photosynthetic capacity was significantly enhanced.The apparent quantum efficiency （AQY） and carboxylation efficiency （CE） were increased by 24% and 5%,indicating that their ability to transform light energy and utilize CO₂ was enhanced.Net photosynthetic rate at light saturation（LSPn）,the light saturation point（LSP）,net photosynthetic rate at CO₂ saturation（Amax） and CO₂ saturation point （CSP） were significantly increased by 49%,36%,27% and 7%,indicating that their light and potential were significantly increased.Absorption of light energy per unit area（ABS/CSo）,capture the light energy per unit area（TRo/CSo） significantly increased 8% and 13%,respectively, to explain the absorption and the ability to capture light energy obviously enhanced; probability of trapped excitons moving electrons into the electron transport chain beyond Q^-_A（Ψ_o）,quantum yield for electron transport(Ψ_Eo) significantly increased by 39% and 46% respectively, in J point relative ariabvle fluorescence intensity（V_j） significantly reduced 40%,explain the electron transfer ability increased significantly.The actual optical quantum yield Y（Ⅱ）of PS Ⅱsignificantly increased by 44%,and the regulatory energy dissipative quantum yield Y（NPQ）of excitation energy significantly decreased by 21%,indicating that it reduced energy loss and used more energy for photochemical reaction.Inoculation of AMF can significantly increasing the content of chlorophyll in‘Fengdan’,and enhancing the ability of absorbing and capturing light energy,improving the electron transfer efficiency and reducing the energy loss,thus improving light energy conversion and CO2 fixation of Paeonia ostii ‘Fengdan’plants,and further improve the photosynthetic capacity and promote its growth.