【Background】At present, the alien invasive weed—Ambrosa trifida L.is rapidly spreading in China, with grave consequences on economy, ecology, and human health. 【Method】In order to study the influence of the rust (Puccinia xanthii f.sp.ambrosiaetrifidae) infection on photosynthetic characters of giant ragweed, photosynthesis rate (Pn), transpiration rate (Tr), vapor deficit at the leaf surface (Vpd), intercellular CO2 concent rations (Ci), and stomatal conductance (Gs) were measured on individuals with photosynthesis measuring system of (CIRAS1). 【Result】After infection with the rust, Pn increased, while Ci, Tr, Vpd, Gs decreased in the leaves from 1 to 4 days after the disease symptoms appeared. During this period, disease severity rised from one to three degree. From 5 to 9 days after the disease apparition, Pn, Tr, Vpd, Gs decreased, meanwhile, disease severity rised from grade four to grade five. 【Conclusion and significance】We found clear evidence that P.xanthii f.sp.ambrosiaetrifidae disturbs the photosynthetic ability of ragweed leaves. This agent merits further study as a potential biological control organism of the giant ragweed.