The flash flood forecasting in small basin is one of the hot and difficult topics in hydrology research. Given the important effect of the soil macropore in runoff formation process, a macropore flow module was added in the HBV model via variable area approach. A case study containing six flood events was operated within Huangnizhuang Basin using the improved HBV model during 2010 and 2015. The results showed that the improved HBV model performs well in Huangnizhuang Basin with the forecast accuracy of the six flood events is grade B or higher, which indicates that the improved HBV model is suitable for the similar basin characterized by short-lived high intensity flash flood. This study can also provide important reference for mountainous basins flood control decision over the upper Huaihe River.