‘Sponge city’ projects are booming all cross China over recent years. The term ‘sponge city’ emphasizes the elasticity of stormwater, but current ‘sponge city’ analyses have yet examined the large-scale horizontal processes and thus failed to systematically understand the dynamics of the stormwater ‘source-sink’. Taking the inner flooding mechanism of the North Canal Basin as an example, this paper introduces the ‘source-sink’ landscape theory in order to analyse the ‘source-sink’ dynamics during the stromwater runoff process. The study selected a series of key landscape factors, and applied the minimum cumulative resistance model to calculate the value of the stormwater runoff stimulus and the surface landscape resistance. The results demonstrate that in the North Canal Basin, the initial ‘source’ runoff locates in mountainous areas, including Mount Mang, Mount Baiwang, and Mount Yangtai. Under natural drainage, the sequences of the ‘source’ landscape types that generate runoff are:forest land > public construction land > road > industrial and facility land > residential area > wasteland > urban green space > farmland. Overall, the construction land has good rainwater absorption capacity with a 5-year rainfall return period, but becomes to be the ‘source’ after 10-year rainfall return period. Within the non-construction land categories, the forest land has the strongest ‘source’ effect even at a 5-year rainfall return period, and the agricultural land has the strongest ‘sink’ effect all the time. The urban green space gradually becomes source after 20-year rainfall return period, the rainwater absorption ability of green spaces warrants further exploration. Waterlogging points concentrate in the north and the west, but scatter in the south and the east of downtown Beijing. As the rainfall intensity increases, the runoff spreads out from the west to the east and from the north to the south. Since the floods in Beijing are probably started from mountain runoffs, practical approaches and strategies are urgently needed to solve the runoff problem caused by subalpine forests in the North Canal Basin.