Abstract: To explore the reasons for the increase in annual precipitation in the Haihe River basin since the 2010s, this study uses multi-source data including measured precipitation, drought/flood severity levels, and reanalysis datasets. Through methods such as wavelet analysis, water vapor balance calculations, and WRF numerical simulations, the study examines the periodic characteristics of precipitation, changes in external water vapor transport, and the impact of diverted water on precipitation. Results show that the Haihe River basin has a 33-year period of precipitation, with the most recent wet phase beginning in 2013. During this phase, annual precipitation increased by 80.0 mm compared to the previous dry phase (1997—2012). From 2013 to 2022, summer water vapor transport through the northwest, southeast, and southwest channels increased by 0.35, 0.29, and 0.26 kg/(m·s) respectively, leading to a total annual precipitation increase of 76.7 mm, accounting for 95.9% of the total increase. Additionally, from 2015 to 2022, the first phase of the South-to-North Water Diversion Project's Middle Route added an estimated 3.3 mm to the annual precipitation, accounting for 4.1% of the increase. This research is of significant importance for understanding the evolution of precipitation in the Haihe River basin and ensuring the long-term security of regional water resources.