The effects of different CO₂ concentrations on the growth and development of invasive plants have been studied intensively, but less scientific interest has been focused on the invasive plant Spartina alterniflora, which has spread rapidly in recent years in the coastal ecosystems of China. In this study, the photosynthetic characteristics under different CO2 concentrations for Spartina alterniflora and for an indigenous plant Phragmites australis at the saltmarsh of Chongming Dongtan，Shanghai were measured and compared during the 2004 growing season. The photosynthetic indices were determined using a LI-6400 portable photosynthesis and accessories CO₂ control system (Li-cor，Inc. USA). The main experimental results are summarized below: (1) With the increase of control CO₂ concentration, the photosynthetic index of net photosynthetic rate and dark respiration rate in Phragmites australis and Spartina alterniflora were raising and falling respectively; (2) Spartina alterniflora had a different photosynthetic/respiration-CO₂ concentration response mechanism, compared with the indigenous plant Phragmites australis, where Spartina alterniflora had a significantly higher apparent carboxylation efficiency (CE) and respiration rate (R), which in turn were more sensitive to CO2 influences; (3) The photosynthetic indexes net photosynthetic rate (P_n), leaf stomatal conductance (G_s) and transpiration rate (T_r) of Spartina alterniflora had more photosynthetic physiological acclimations than Phragmites australis under the 400μmol/mol CO2 concentration; however, Spartina alterniflora showed a stress response under the 1000μmol/mol CO2 concentration. These results could be helpful in explaining the physiological and ecological mechanisms of Spartina alterniflora with respect to its growth rate, productivity and strong competitive ability.