This paper gave a preliminary analysis of the characteristic and vari ation regularity of runoff of the Yangtze river in low-flow period (from Dec. to Mar.), and the effects on the quality of the water of the Huang pu river after the intrusion of sea water.Although rich in water resource the Yangtze river hasn't much discharge during the low-flow period. The total runoff in low-flow period occupies only fifteen per cent of the annual total runoff. The mean monthly discharge in low-flow period of normal year of 50% frequency is below 15000 M³/sec. The mean monthly discharge of 75% frequency of dry year in Jan.and Feb, is less than 10000 M³/sec. Salinity varies with the quantity of the flow which comes from upstream. It becomes highly sensitive in river-mouth area when the discharge in Da Tong station is 10000 M³/sec. The salinity in river-mouth area will increase repidly if the discharge in Da Tong station is less than 10000 M³/sec. As a result, serious impacts will happen to the water supply both of Shanghai city and the industrial and agricultural bases in river-mouth area. For the planning of water supply in the Yangtze river basin,it is necessary to keep the discharge through Da Ton station above 15000 M³/sec, or at least not less than 10000 M³/sec.