Abstract: The aim of the experiment is to study the effects of nitrogenous fertilizer and PP333 on the seedling growth, transplanting quantity and grain yield of mechanical transplanting rice in wheat-rice double cropping area. The seedling quality and grain yield were analyzed by conventional physiological techniques, the roots twisting power was measured by spring balance, the transplanting quantity was evaluated by GB/T 6243-2003. The results showed that, in the over-optimum especially serious over-optimum stage of mechanical transplanting rice seedling, the seedling quality decreased rapidly, i.e. the plant height increase rapidly while the dry weight per 100 plants and root vigor decrease rapidly. Meanwhile, the MDA content of seedling leaf increased in line. Soaking seeds with 100 mg/L PP333 and spraying seedling leaf with 250 mg/L PP333 in 2.5 leaf age combining the application of 6 g/m2 nitrogenous fertilizer in 1.5 leaf age, the seedling quality was better than other treatments, besides, the rate of empty-hole was 8.5%, the grain yield can reach to 9.95 t/ha. Thus, applying suitable nitrogenous fertilizer in 1.5 leaf age combined with two-time PP333 can help to keep good initial transplant vigor and transplant quality, ensuring to achieve the high grain yield.