The elevated CO_2 concentration is one of main features of global climate change. As the raw material of photosynthesis, CO_2 has an important influence on the growth and development of plants. Rice is one of the most important crops in the world, the study of crop response to different levels of CO_2 concentration is of great significance for guiding agricultural production and ensuring food security. To investigate the effects of elevated CO_2 concentration levels on photosynthetic characteristics of rice, field experiments with simulating elevated CO_2 concentration were conducted by open-top chambers (OTCs) using a CO_2 concentration automatic control system to achieve different levels of elevated CO_2 concentration. The experimental treatments included the ambient CO_2 concentration (CK), 40 mumol·mol~(-1) above ambient CO_2 (T_1), 200 mumol·mol~(-1) above ambient CO_2 (T_2). The data of net photosynthetic rate (Pn), intercellular CO_2 concentration (Ci), stomatal conductance (Gs), transpiration rate (Tr), water use efficiency (WUE) were measured using a Li-6400 portable photosynthesis system. The key photosynthetic parameters including maximum photosynthetic rate (Pn_(sat)), carboxylation efficiency (CE), light respiration rate (Rp), CO_2 compensation point (CCP) were fitted and estimated according to the exponential equation model. The nitrogen content of leaves was determined by using the Element Analyzer. The results showed that Pn increased rapidly under the relatively lower CO_2 concentration ranges and then increased slowly to the maximum value. When CO_2 concentration arrived at the saturation point, Pn tended to be stable. At the CO_2 concentration of 600 mumol·mol~(-1), compared to CK, T_2 treatments significantly reduced Pn by 44.0% (P<0.01), 43.4% (P<0.01) and 49.1% (P<0.01) at the jointing, heading and milking maturity stages, respectively. At the CO_2 concentration of 800 mumol·mol~(-1), compared to CK, T_2 treatments significantly reduced Pn by 4.9% (P=0.506), 12.7% (P=0.167) and 16.6% (P=0.220) at the milking maturity stage. At the CO_2 concentration of 1 000 mumol·mol~(-1), compared to T_1, T_2 treatments significantly reduced Pn by 21.5% (P<0.05) at the milking maturity stage. The photosynthetic acclimation appeared in rice leaves under T_2 treatment. Compared to T_1, T_2 treatments significantly reduced Pn_(sat) and CE by 21.3% (P<0.05) and 29.1% (P<0.05), respectivley. The total photosynthetic rate was significantly and positively correlated with the leaf nitrogen content (P<0.01), and increased with the increase of the leaf nitrogen content. WUE of different treatments increased with the elevated CO_2 concentration, CK, T_1 and T_2 treatment had no significant effect on WUE at the same CO_2 concentration.