To uncover the diurnal dynamic character of photosynthesis and its impacting factors of Arundo donax var. versicolor in ecological ditch, net photosynthetic rate (Pn), photosynthetically active radiation (PAR), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci), leaf temperature (Tl), atmospheric temperature around the leaves (Ta) and transpiration rate (Tr) were measured in clear days using portable photosynthetic system (the model type is CID–340). The results showed: ①The diurnal Pn curve of Arundo donax var. versicolor presented a single-peak, which occurred at about 3 pm with a value of 19.76 μmol/(m2•s). ②Results of stepwise regression indicated that Tr, Ci, Tl and Ta took an important effect on Pn. ③Results of partial correlation analysis and path analysis indicated that Ci and Tr were two important factors influencing to Pn, and Ci had larger impact on Pn in comparison with Tr.