River-lake system interconnection demand is one of the key elements among the research of river-lake system interconnection strategy.It reflects the demand of the social-economic development on water diversion,flood regulation,eco-environment improvement and the maintenance of the health of river-lake system.It is based on the inherent ecological properties that a drainage system can provide various water ecosystem services to a river-lake system.Since the river system,river system interconnection demand, the water ecosystem services,and river system interconnection engineering are closely interconnected,the methods which have been adequately studied for assessing the water ecosystem services can be applied to the assessment of river system interconnection demand.So methodologies for evaluation of river system interconnection demand are established by applying the water equivalent method.With a long historical practices of river system interconnection in Lake Taihu watershed,which is a typical and emergent floodplain river network area in eastern China,demanding on Lake Taihu river system interconnection is assessed in this study.The main demand includes water resource diversion,flood regulation,water purification,and habitat sustainment.Purification water demand per year of retained pollutants in the lake is 2.85×10¹⁰ m³/a,ranked the first place.Water resource diversion demand is 3.0×10⁹-4.2×10⁹ m³/a,ranked the second place,while the demand of flood regulation of the lake is decreased by 7.2×10⁸-10.7×10⁸ m³/a.Purification demand of the water from the lake is 8.14×10⁹ m³,and is consumed once only;the habitat maintenance water demand is 3.58×10⁹ m³.The results show that water environment improvement demand is tremendous.To ease the stress,countermeasures such as control of the pollutant entering into the lake,desilting and dredging the lake,and ecological restoration of the lake water need to be applied firstly,then the diversion of clean water as an assisting way.There are different corresponding interconnection ways for each demand,and it is necessary to confirm the ratio of each interconnection project.