By a solution culture experiment, this paper studied the effects of excess Mn on the growth, chlorophyll content, chlorophyll fluorescence parameters and photosynthesis of cucumber under different light intensity. The results indicated that excess Mn inhibited plant growth, which was more obvious under high light intensity than under low light intensity. The primary maximum photochemical efficiency of PSII (v/Fm), quantum efficiency of non-cyclic electron transport of PSII (phiPSII), and photochemical quenching (qP) were significantly decreased in excess Mn treatment under high light intensity, while no significant effects on Fv/Fm and qP were observed under low light intensity. Excess Mn, particularly under high light intensity, decreased net photosynthetic rate (Pn) and stomatal conductance (Gs). Excess Mn increased intracellular CO2 (Ci) under high light intensity and decreased Ci under low light intensity, while stomatal limitation value (Ls) was just reverse to Ci. It could be concluded that the decrease of Pn in excess Mn treatment was not resulted from stomatal limitation under high light intensity, but was true under low light intensity.