Abstract: Understanding key factors affecting sediment transport and the threshold of sediment transport in a heavy rainfall is crucial to preventing sediment disasters and reducing soil erosions. In this study, the small watershed of Hemingguan, a purple soil region in Sichuan Province, is chosen as a typical area for research. The methods of statistical classification and comparison are used to analyze sediment transport data in recent 10 years. The influence of sub-rainfall and rainfall intensity and duration on the sudden increase of sediment transport is studied. The key factors and the threshold leading to the sudden increase of sediment transport are clarified. The results indicate that, in a typical Sichuan purple soil area, key factors for a sudden increase in sediment transport are sub-rainfall, rainfall intensity and the maximum runoff modulus. The threshold for rainfall intensity is 5.3 mm/h. The threshold for rainfall is 130 mm, and the threshold for the maximum runoff modulus of slope land is 6 000 dm³/(km²·s), based on tillage. The threshold for the maximum runoff modulus of slope land is 3 000 dm³/(km²·s), based on planting grass and trees.