Abstract: Southwestern China experienced a typical compound heat–drought event between mid-August and mid-September 2024, which significantly impacted the local economy and livelihoods. In this region, such compound events are related to intraseasonal oscillations (ISOs) originating from different areas. However, conventional statistical methods fail to distinguish the specific contributions of various ISOs to these extreme conditions. Using atmospheric-circulation diagnostics and numerical simulations using the Weather Research and Forecasting model with varying-ISO-signal-driven boundary forcings, this study reveals the following: the ISO signal induced by a mid-latitude westerly wave train in the upper troposphere increased the regional average temperature by 1.331°C in Southwestern China, primarily affecting the northern part of the region. This ISO signal accounted for 62.8% of the overall compound heat–drought event. The ISO signal associated with a middle-tropospheric teleconnection wave train increased regional average temperatures by 0.768°C. This impact is mainly concentrated in the eastern Sichuan Basin and most of Chongqing, accounting for 37.2% of the event. The development of this compound event was predominantly influenced by a mid-latitude westerly wave train from the northwestern boundary. This promoted the significant expansion of the South Asian High over Southwestern China, resulting in strong positive geopotential height anomalies. Simultaneously, warm advection due to the westward extension and northward shift of the Western North Pacific Subtropical High, coupled with positive tropospheric geopotential height anomalies, contributed to regional temperature increases and decreased soil moisture. Overall, this study enhances the understanding of the mechanisms driving compound heat–drought events in Southwestern China, providing a foundation for improving subseasonal-prediction capabilities.