The change rule of root growth and respiratory metabolism were analyzed under different treatment times(1,3,5,recovery 7 d) and different levels(root flooded and half submerged) in pakchoi (Brassica campestris ssp.chinensis Makino).(1)Waterlogging stress inhibited the plant roots growth of pakchoi,ruesulted in the decrease of root fresh,root length,and root activity,and the reduction of half submerged was higher than that of root flooded.(2)Comparing with the control,waterlogging stress reduced the root malate dehydrogenase (MDH) and succinte dehydrogenase (SDH) activities significantly,increased the lactate dehydrogenase (LDH) and alcohol dehydrogenase (ADH) activities.The impact of semi-submergence of plants was more serious than that of submergence of roots,and 5 d and 1 d treatments have significant difference.(3)After the waterlogging stress,the lactic acid fermentation way of ‘Xinaiqing’ was weaker than that of ‘Xinxiaqing’.By contrast,alcoholic fermentation of ‘Xinxiaqing 2’ was stonger,causing the lactic acid accumulation in the root is more than that of ‘Xinaiqing’,and the degree of cytoplasm acidification was seriously reducing the tolerance to flooding stress.Obviously,after the waterlogging stress,the aerobic respiration of pakchoi was inhibited while the anaerobic respiration was accelerated.With the extension of flood time,the degree of inhibition on root respiration metabolism was more serious,eventually lead to restrain root growth.