<p>The Sanjiangyuan region, known as the “Water Tower of China”, is the source of the Yangtze, Yellow, and Lancang rivers. The diurnal variation of summer precipitation in this region is essential to the regional water cycle. In this study, we utilize the Tropical Rainfall Measuring Mission (TRMM) 3B42 Version 7 and the Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) datasets to investigate the diurnal patterns of summer precipitation in the Sanjiangyuan region. Furthermore, we explore the underlying causes of these diurnal variations in precipitation from the perspective of moisture. The results show that: (1) the climatological summer precipitation in the Sanjiangyuan area follows a diurnal pattern, with less rainfall during the day and more at night, peaking at 20 Beijing Time Coordinate. (2) The diurnal variation in summer precipitation in the Sanjiangyuan area is mainly caused by the diurnal cycle of horizontal moisture transport, with precipitation peaking when moisture transport is at its maximum. (3) The diurnal variation in horizontal moisture transport in the Sanjiangyuan area is closely linked to the moisture balance at the region’s eastern and northern boundaries. Boundary layer inertial oscillations primarily govern the diurnal moisture budgets along the eastern boundary, while mountain-valley wind circulations predominantly control them along the northern boundary.</p>