In 2021, heavy rainfall from mid-August to late August caused severe floods in China’s Yangtze River Basin. Our analysis reveals that intraseasonal (30-80 day) oscillation contributed to more than half of the total rainfall anomalies. A concurrent dipolar northward propagating boreal summer intraseasonal oscillation (BSISO) episode coincided nicely with this extreme event, but it alone cannot explain its extremity. Through nonlinear K-means cluster analysis, we identify a new type of BSISO, termed Slanted Northward Propagating (SNP) BSISO. The 2021 event was categorized as one of eight cases of SNP BSISO characterized by a southwest-northeast orientated and westward-displaced convection structure over the northwestern Pacific and South China Sea. Additionally, the development of La Niña significantly influenced the SNP BSISO, generating specific wind patterns over the northwest Pacific and enhancing the moisture over the Maritime Continent, which facilitates the westward expansion of western Pacific subtropical high and occurrence of 2021 event.