Abstract: Formation mechanism and moisture characteristics of an extreme precipitation event (maximum hourly rainfall intensity of 85.7 mm) in Gansu Province on 3 August 2025, were investigated using 0.25°(latitude)×0.25°(longitude) hourly resolution ERA5 reanalysis data from ECMWF and surface station hourly precipitation observations. Methods such as synoptic diagnosis, kinetic energy and vorticity budget analyses, and Lagrangian moisture tracking were employed. Results indicate that the extreme precipitation event occurred in two stages under a stable circulation background. In Stage I, precipitation was primarily triggered by a low-pressure trough located on the southwestern flank of a saddle-shaped flow pattern in the lower troposphere. A strong pressure gradient force from the high-pressure area to the low-pressure trough maintained a persistent easterly wind, whose convergence against the high topography of the northeastern Tibetan Plateau was the main dynamic lifting mechanism during this stage. In Stage II, precipitation was associated with a quasi-stationary mesoscale vortex. The generation and maintenance of this vortex were predominantly governed by the stretching term associated with low-level convergence. Moreover, upward cyclonic vorticity advection contributed to its persistence. Lagrangian moisture budget analysis revealed that the moisture supply for this extreme precipitation event was dominated by local evaporation and terrestrial moisture recycling (contributing approximately 40.4%), with secondary contributions from western China–Central Asia and eastern China. Conversely, direct contributions from remote oceanic moisture sources were negligible. These quantitative findings highlight the unique terrestrial-dominated moisture transport properties of extreme precipitation in inland northwestern China. The results provide scientific insights to improve understanding, forecasting, and early warning capabilities for such extreme precipitation events in the region.