The effects of land-use/land-cover change (LUCC) on the watershed flood process have become one of the major hydrological concerns in the world. A change in the flood process leads to the alteration in flood design and control system, which has caused serious problems to human life and urban security. In this study, we take Jiaokou Reservoir watershed as a sample area to analyze the LUCC effects on storm floods of different return periods based on the hydrological model (HEC-HMS) and GIS-related support. Jiaokou Reservior is located in the west of Ningbo city, Zhejiang Province, and is controlled by the plum rains and typhoon storms with various return periods during rainy seasons. Due to rapid economic development and intensive human activity, the land utilization pattern has changed in recent years. The spatial data of land use of the study area during 1985-2003 was obtained from Landsat TM (Thematic Mapper) image data, and the images from different periods were interpreted and overlapped to setup the transfer matrix of the land-use types. The land-use types were dominated by forest land, bush grassland, and cultivated land, which covered 92% of the watershed. The main LUCC in this area is the conversion of forest to bare land, grassland, and agricultural land. The analysis of the LUCC impacts on flood shows that (1) during 1985-2003, 3%-30% of the forest land was changed to bare land, bush grass, or agricultural land, and the LUCC has different effects on floods of different return periods. Both flood runoff and peak increased for all flood magnitudes, but the runoff changed at a higher range as compared to the flood peak. (2) Low magnitude flood was found to be more sensitive to the LUCC, the increase of the total runoff and flood peak of a 5-year flood increased by 3% and 7.6%, respectively, while those of less than 2-year return period floods increased by 5.41% and 11.91%, respectively. From the perceptive of flood classification, the flood return periods of 100, 50, and 25 years were reduced by 15, 6, and 2 years, respectively, during the study period. (3) Different land-use change patterns lead to flood variations. Along with the forest being characterized by a change ratio of 10%-50% since 1985, forest-bare land-use change pattern impacts flooding most intensively, and the forest-grassland change pattern has stronger effects than the forest-agricultural change pattern. Compared with a simulated flood in 1985, when forest was converted into bare land with a ratio of 10%-50%, the flood peak increased by 0.88%-4.3% and the total runoff increased by 1.61%-7.91% in the 100-year-return-period flood. While, the widened range of the flood peak and total runoff under the same condition were 2.18%-10.67% and 4.15%-20.62%, respectively, in the 5-year-return-period flood. As the change ratio gets bigger, the difference in impacts of different patterns of forest transformation to other lands on floods became more distinct, especially to the low-magnitude flood. The results of this study can provide scientific supports for the flood control and water resource management.