[Objectives]Understanding how the photosynthetic system Ⅱ would respond to the increased UV-B radiation is critical to evaluating the adaptability of rice under global change context. As the chlorophyll fast fluorescence induction kinetic curves(OJIP)and parameters were widely used to describe photosynthetic system Ⅱ,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 rice variety'Shennong 265'was used and exposed to either natural light(CK)or enhanced UV-B radiation. The treatments were set as T1,T2 in which the UV-B radiation was increased by 1.05 and 2.10 W·m~(-2)respectively. 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 make the vivo measurements of each period of flag leaf,depending on the measurement item,each treatment repeated 3 or 6 times. [Results]When the UV-B radiation increased,the Point P decreased on the curve of OJIP in PSⅡ donor in each period.The increase of UV-B radiation had a significant effect on the energy distribution of PSⅡ reaction center:the activity parameters of the unit reaction area in the growth period of the flag leaf showed a decreasing trend,which indicated that UV-B radiation inhibited it,leading to the activity parameters of the unit reaction center increasing in different degrees. The F_v/F_m,qP,rETR of flag leaf decreased,and T2 inhibited them greater than T1. Performance indiced in growth period decreased,especially on booting. The inhibition effects of T2 was greater than that of T1. [Conclusions]The enhanced UV-B radiation reduced light energy of absorbing,heat dissipation,light energy of capturing,the quantum yield of electron transport and active reaction centers of the flag leaf,but promoted the the growth of the activity parameters in unit activity center.When the UV-B radiation increased,the primary energy conversion efficiency,photochemical quenching coefficient,photosynthetic electron transport relative rate and the performance index(which is in basis to absorb light energy)decreased.