Short-duration heavy rainfall causes severe urban flooding, threatening urban security and socio-economic development. The lower reaches of the Yangtze River are one of the regions with the highest frequency of short-duration heavy rainfalls in China. In this study, hourly stable isotope compositions in water vapor (δ¹⁸O_v and d-excess_v) are analyzed for nine short-duration heavy rainfall events during the summer monsoon season (June to September) from 2013 to 2021 in Nanjing, eastern China. The circulation patterns that lead to these events can be divided into four types: tropical cyclone, low-pressure vortex, cold front, and western North Pacific subtropical high. During these events, δ¹⁸O_v is enriched, ranging from −18.8 ‰ to −13.7 ‰ with a pre-storm increasing trend. This is largely caused by strongly isolated meso- and small-scale convections and the close proximity of oceanic moisture sources. The d-excess_v shows three different variation patterns during rainfall events: increasing, decreasing, and irregular fluctuations. They each correspond to the increasing contribution of terrestrial moisture from eastern China, proximal oceanic moisture from China’s offshore waters (including the South China Sea, East China Sea, and Yellow Sea), and the mixing influence of these two moisture sources. Multiple patterns of the d-excess_v variations reflect both importance of terrestrial moisture from eastern China and oceanic moisture from China’s offshore waters for summer short-duration heavy rainfall events in eastern China.