Abstract: The cumulation photosynthesis effect in the condition of different irriganons on the paddy has the different resuits．The contrast of colony photosynthesis efficiency after heading between different irrigations in the paddy is notable but very little in the early paddy．The midday depression always appears regardless of the sunny days or cloudy days in the condition of normal irrigation．The photosynthes is reaches the peak values at 9 o’clock and 15 o’clock in the cloudy days，and at 11 o’clock and 17 o’clock in the sunshine days．Data indicate that the air humidity and the concentration of C0₂ are the two dominant environmental factors affecting the photos ynthesis peak value．The peak value of Tianguan appears ahead of time by the other measurements in the sunshine days，but it has the nearly reversed result in the cloudy days．of the four kinds of irrigation，Tianguan has the weakest photosynthes is velocity，Yanguan has the strongest，and Peiguan stands in the middle．