In order to find out the impact and mechanism of orchid mycorrhizal (OM) fungi on photosynthesis of Dendrobium officinale, we investigated the effect of OM fungi on the growth of D. officinale seedlings using a pot experiment. The chlorophyll content, photosynthetic characteristics, chlorophyll fluorescence parameters, and the expression of pepc gene were analyzed. The results were as follows: (1) OM fungi could improve the growth of D. officinale seedlings. The height of plants, mass of roots, mass of shoots and total biomass of D. officinale seedlings were of 1.21, 1.54, 1.71 and 1.68fold, respectively, compared to control. Chlorophyll content, net photosynthetic rate (P_n), stomatal conductance (G_s) and transpiration rate (T_r) of D. officinale seedlings increased too. (2) The potential photochemical efficiency (F_v/F₀), photochemical maximum efficiency (F_v/F_m), the photochemical quenching coefficient (q_P), nonphotochemical quenching coefficient (q_N), quantum yield of electron transport (Yield) and apparent photosynthetic electron transport rate (ETR) in leaves of _{D. officinale} seedlings inoculated with OM fungi were higher than that of control. (3) OM fungi could enhance the expression level of pepc gene, PEPC activity, and promote photosynthetic carbon assimilation of D. officinale. Therefore, the development of D. officinale mycorrhiza could improve photosynthesis, the expression level of pepc gene and the growth of D. officinale seedlings.