Abstract: Analysis of "four-water" transformation lays the foundation of the study of water resources problems such as water resources assessment and water cycle changes. In this study, a modified version of the MODCYCLE distributed hydrological model is established in accordance with the local specifications of Tianjin, and then used to analyze the regularity of "four-water" transformation in Tianjin. The original MODCYCLE model was developed by the Department of Water Resources in China Institute of Water Resources and Hydropower Research, and is able to account for the influence of intense human activity, especially when dealing with complex agricultural systems. The analysis result shows that the process of "four-water" transformation in Tianjin is largely influenced by the artificial collateral water cycle, which can be characterized as the reduction in surface runoff, the increase in soil water content, the overexploitation of groundwater, as well as the efficient use of inflow water and transferred water. The quantity of surface water in Tianjin could hardly meet the local demand for water. Although, the current shortfall in water provision is resolved by using upstream inflow and transferred water from other regions, and the issue of future water shortages could be ensured by the South-to-North Water Diversion Project. However, a strict water resources management system should be implemented to gradually reduce the groundwater consumption and to increase the freshwater discharge into the sea.