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How boreal summer intraseasonal oscillation (BSISO) modulates the probability and spatialdistributions of extreme rainfall occurrence over populous Southeast China was examined, usingthe newly proposed BSISO indices and two high-resolution rain-gauge-based rainfall datasets inChina. The probability density function of May-August rainfall in Southeast China is skewedtoward large values in phases 2-4 0f the first component and in phases 5-7 0f the secondcomponent of BSISO life cycle, during which the probability of extreme rainfall events at the 75th(90th) percentile increases by 30-50％ (over 60%) relative to the non-BSISO period. The mostdevastating floods with prolonged extreme rainfall in the Yangtze River Valley and southenChinaover the past half-century occurred coincidently with these BSISO phases. Moisture budgetindicates that the favorable environment for rainfall extreme is associated with southwesterlymoisture convergence over Southeast China, while the moisture advection contributesinsignificantly. This study suggests a potential for monitoring and probabilistic prediction ofextreme rainfall events in Southeast China based on the real-time BSISO indices.