Abstract: Take variety GR911 as testing material, spray 500mg/L and 1000mg/L paclobutrazol (PP333) solution on leaves of cassava during expanding period of root tubers. It was found that PP333 can shorten cassaca's plant height, decrease leaf area, but increase the weight of unite leaf, content of photosynthetic pigment, transpiration rate and stomata conductance, increase the net photosynthetic, root/crown ratio， improves the photosynthetic material accumulation and the yield.