Abstract: The root and photosynthesis physiological characteristics of different populations of two hybrid rice Honglianyou 6 and Liangyou 1193 after flowering stage under SRI were studied. Results showed that root density was higher in small population than those of lager population, but converse to the root activity. The chlorophyll contents of flag leaf were not different at flowering stage, but which was significantly higher than that of in low yield population, so the range of reduction per days was small in high yield population. The net photosynthesis rate of flag leaf in high yield population also was significantly higher than that of low yield population in reproductive stage. The yield superiority might result from its enhancement of physical activity of plant favorable to the grain development in reproductive stage