The net photosynthetic capacity (Pn), respiration rate (R), stomatal conductance (gs), transpiration rate (Tr), carboxylation efficiency (CE), maximum apparent quantum yield () and photosynthetic water use efficiency (WUE) of Mikania micrantha was measured under different conditions. These included a variety of habitats, different leaf sizes, leaf ages and leaf orders, decumbent or climbing habits, presence or absence of parasitism by Cuscuta campestris, and was carried out in both the vegetative (summer) and reproductive (winter) seasons. Further comparisons with other herbaceous, woody and liane species in the same region (during the vegetative season) or population (in the reproductive season) were also carried out. The following conclusions were drawn from the results. (1) Under diverse conditions, parameters of CO2 and H2O exchange by M. micrantha varied from several-fold to ten-fold, thus showing large plasticity in acclimatizing to different microenvironments. On average, the Pn of M. micrantha in both the vegetative and reproductive seasons was equivalent to that of the woody species in the same region or population was slightly lower than in liane species, and much lower than in herbs. This finding indicates that the strong invasive ability of this weed might not be related to the photosynthetic capacity per leaf area, but rather may be due to other mechanisms such as its strong ability to acclimatize to different habitats on account of its photosynthetic plasticity. (2) The photosynthetic capacity of M. micrantha on the forest floor was much lower than that in the glade, while plants on the forest margin had the highest Pn. Moreover, a contrasting experiment also demonstrated that M. micrantha plants growing in shady sites photosynthesized at a much lower rate than plants in sunny sites. These results reflect the ecological characteristics of this weed and that it demands strong light for vigorous growth. Thus, reducing light infiltration through the modification of forest structure as well as increasing crown density should be an effective ecological method for controlling this weed. (3) Data (whole data in present paper) statistics showed that Pn of this weed centralized in the range of 2-10 μmol m-2 s-1 (70% of total data), gs in the range of 0.05-0.45 mol m-2 s-1 (73%), Tr in the range of 1-5 mmol m-2 s-1 (66%). About 64% of total CE was in the range of 0.01-0.05mol.mol-1. Near half R was in the range of 0.5-1.5 μmol m-2s-1, while 66% of the data was in the range of 05-2.5 μmol m-2s-1.The distribution center of (77%) was 0.04-0.08 mol.mol-1. These frequency distribution analyses reflect all measured data is in a normal or bias-normal distribution and these data made it possible for inter-species comparison on a statistical base. (4) Similar regulating mechanisms on Pn and WUE of this weed were found in different habitats, leaf ages, and growth status. Pn was mainly regulated by CE and secondly by gs, but no significant correlation was found between  and Pn. WUE was mainly controlled by Pn, while Tr made a relatively low contribution. The WUE was maintained at a constant level with changes in gs. Overall, the data of gas exchanges in this paper of this weed may strengthen the base for controlling the invasion of this species.