[Objectives]Understanding how the photosynthesis would respond to the increased UV-B radiation is critical to evaluating the adaptability of rice under global change context. This paper aims to characterize the changes of these parameters of japonica rice when the UV-B radiation is enhanced. [Methods]In this study,the japonica varieties'Shennong 265'was used and was exposed to either natural light(CK)or enhanced UV-B radiation. The treatments were set as T1 and T2 in which the UV-B radiation was increased by 1.05 and 2.1 W·m -2 respectively,with each treatment repeated 3 or 6 times. Selecting the seedling transplant into the nonporous test barrel,UV-B radiation time is from 08:00 to 16:00,without in rainy days until the growth. We made the vivo measurements each period of flag leaf,depending on the measurement item:the photosynthetic electron transport and photosynthetic light response parameters,including J_c,J_o,apparent quantum yield(AQY),maximum photosynthesis rate(P_(max))and the daytime ecosystem respiration(R_d). [Results]When the UV-B radiation increased,the J_c and J_o of flag leaves decreased,while J_o/J_F and J_o/J_c increased. This indicated that enhanced UV-B reduced the quantity of photosynthetic electrons which was used to get into the progress of carbon reduction. The UV-B radiation reduced the V_o and V_c,but increased V_o/V_c of the flag leaves in each growing period. It was positively related to the intensity of UV-B radiation which indicated that enhanced UV-B reduced the carboxylation rate of Rubisco. Generally,the increased UV-B radiation had no effects on the trends of light response curves. The LSP decreased because of the increased UV-B radiation in each growing stages except for maturation stage. In filling stage,the decrease amplitude reached its maximum. The LCP increased in each growing stages except for filling stage. The increase amplitude got to its maximum in booting stage. In addition,the AQY decreased. The P_(max) decreased significantly and the inhibition effects of T2 was greater than that of T1. Except for maturation stage,the R_d increased,especially in filling stage. [Conclusions]When the UV-B radiation increased,the photorespiration energy consumption of the flag leaves increased,and the quantity of photosynthetic electrons which was used to get into the progress of carbon reduction decreased. The proportion of the photosynthetic electron on oxidation reactions of Rubisco increased. The enhanced UV-B radiation inhibited the photosynthetic electron transfer to the carboxylate direction,resulting in the decrease of the transfer rate of carboxylation. With the same PAR,the photosynthetic rate decreased and the photosynthetic capacity weakened when the UV-B radiation was enhanced.