Abstract: Because of the excessive development and utilization of water resources, freshwater inflow to estuaries has sharply decreased in the Haihe River basin in recent years, thus causing serious ecosystem degeneration. The salinity is chosen as a key environmental factor of the estuarine habitat. The relationship between the salinity and runoff is simulated using 1D mathematical model that has coupled a hydrodynamic model with a salinity model. With the annual average maximum salinity value obtained from Tanggu station taken as the control standard in the 1970s, the minimum ecological water requirement is calculated for the Zhangweixin River estuary under conditions of the standard. The research results show that the minimum ecological water requirement for the Zhangweixin River estuary is 3.19 billion m3. The annual average freshwater inflow is 1.55 billion m3 since 1980s, and the minimum ecological water requirement cannot be satisfied by 84% of the years′ annual freshwater inflow in the Zhangweixin River estuary. The calculated results can provide reference for the basin ecological regulation and estuarine ecosystem restoration. Considering that there are many factors such as topography, runoff, tide, wave, sediment and salinity affecting the ecological water requirement and more complex kinematic process in the estuarine environment, it is necessary for us to simulate and analyse the response of the key environmental factors to freshwater inflow by use of hydrology, ecology, hydrodynamics, and geography, and to combine numerical simulation with GIS spatial analysis methodology in the study of the ecological water requirement so as to make the assessment results of the ecological water requirement in estuaries more reliable and practical in the future researches.