This study aims to analyse the comprehensive effects of an ecological water diversion project (EWDP) and its driving mechanism since 2000 in Heihe River Basin (HRB) in Northwest China. We compared the hydrological, ecological, and socio-economic changes in the period of 1985–2015, revealed the relationships between them using linear regression method, and discussed their internal mechanisms referring to the long-term monitoring data, statistics data, field surveys and remote sensing data. We found that the hydrological regime in the region downstream of Langxinshan Hydrological Station (LXS region) in the lower HRB has changed, with an increase of annual discharge as well as obvious differences in the intra-annual distribution of monthly runoff. As a result, the groundwater level in most of the LXS region has risen, but the changes of the chemical characteristics of the groundwater were not significant. Meanwhile, the vegetation has been remarkably restored, and the water and wetland continually expanded suggesting that the degraded environment in the LXS region has gradually been rehabilitated. Additionally, the regional economy has developed rapidly, with a rapid increase of value-added tertiary industry due to the explosive growth of the tourist population. Along with the EWDP, other policies and projects have played an important role in restoring the ecology. The hydrological, ecological, and socio-economic systems have formed feedback loops in the LXS region, but there is still need to coordinate the relationships between water resource, ecosystem and socio-economy by considering water carrying capacity and water production efficiency and thinking of them as a nexus.