Using surface dense automatic precipitation observations, ERA5 reanalysis data, and radar composite reflectivity data, this study analyzes the causes of a heavy rainstorm event over the western Sichuan Basin on July 11, 2023, focusing on the impacts of inertial oscillation on wind fields and terrain–airflow interactions on rainfall. A physical conceptual model of this rainstorm event is also developed. The results show that this precipitation event exhibited three prominent characteristics: a linear distribution of heavy rainfall along mountainous terrain, pronounced nocturnal development, and strong convective activity. Before the heavy rainstorm, the Sichuan region was under the control of the South Asian High, while the basin was influenced by southerly airflow on the periphery of the Western Pacific Subtropical High, with no considerable weather systems affecting the area. Quantitative analysis revealed that the Coriolis force and geostrophic deviation terms showed large variations. While atmospheric advection and vertical transport were weak, inertial oscillation dominated the wind speed increase in the central–eastern Sichuan Basin and eastern Yunnan–Guizhou Plateau. This process led to nocturnal boundary layer jets and clockwise rotation of the ageostrophic wind. The development of southeastward ageostrophic winds at night enhanced the southeasterly flow over the basin. The superposition of jet transport from the eastern Yunnan–Guizhou Plateau and wind speed growth in the central–eastern basin enhanced wind convergence in the basin. The combined effects of the divergence term and vertical transport term promoted the development of cyclonic wind fields. Under the combined influence of airflow in the northern sector of the cyclonic wind field and ageostrophic winds, southeasterly winds persisted over the northwestern basin, playing a crucial role in the formation of nocturnal rainfall. The southeasterly winds transported warm, moist air westward, leading to considerable wind and moisture convergence along the mountains of the western basin. The intrusion of dry air at mid-to-upper levels combined with low-level warm and moist airflow enhanced convective instability. The perpendicular intersection of southeasterly winds with the mountains of the western basin produced strong upslope flow. Orographic lifting on windward slopes and the formation of secondary vertical circulations triggered convection ahead of the mountains. As nocturnal southeasterlies intensified, convergence ahead of the mountains strengthened, and convection continued to develop. The persistent southeasterlies combined with topographic blocking and cyclonic wind field development, generated northerly winds along the western basin margin and easterly winds along the northern edge. Their convergence with easterly/southeasterly flows formed wind shear lines that promoted convective organization. The steep terrain transition from the basin to the mountains and the perpendicular intersection of the southeasterlies with the northeast–southwest-oriented mountains in the western basin favored strong upward motion on windward slopes. Sensitivity experiments conducted using a multisource observation data assimilation system and the WRF model demonstrated that halving terrain slopes reduced precipitation intensity by one order of magnitude. This decrease clearly indicates the enhancement effect of steep topography on rainfall. Under the combined forcing of low-level shear lines and steep terrain, a quasi-linear intense convective system developed along the mountains, forming a linear downpour zone along the western basin’s foothills. With the interaction of persistent southeasterly flow and the unique topography, efficient precipitation persisted, ultimately causing the extreme rainfall event.