Abstract: The overall water quality of Qian-dao Lake in the flood of Year 1996 presents a ‘pollution peak’in the historical tendency diagram,indicating that alien mass pollution is the main source of pollution.According to the comparative analysis of the water quality changes before and after the flood,the effect of the flood on the water quality is limited to the flooding year,and all pollution indexes can quickly resume to the normal after the flood through the cushion and self-purification of the water body.In the flooding year,the orders of the pollution indexes and the load ratio are the same for the sections of Baimufan,Hangtou Island,Jiekou and Santan Island.The total load ratio of these four sections reaches 72% of the whole lake.Fecal coliform and total N have the biggest load ratios of the whole lake, whereas the rest items have very low load ratios and have little effect on the total water quality in the flooding year.The load ratio of fecal coliform reaches 46.6%.It becomes the first pollution factor of the water quality in the flooding year.Total Cd,ammonia nitrogen,and total Pb all demonstrate an abnormal increase.Especially,the indexes of total Cd and total Pb can not resume normally after the flood,indicating a ‘lag effect’of heavy metal pollution caused by the flood.