Spatiotemporal variability of the runoff flashiness index was explored and thereby used to evaluate the effect of humanactivities including dam construction and land use change on runoff variation. The Jiulongjiang River Basin, the medium -sizedcoastal watershed located in SE China, was chosen to analyze the characteristics of the runoff variation with Flashiness Index (FI)and Baseflow Index (BFI) for the north reach and west river of the Jiulongjiang River, based on the forty-year hydrologic data from1967 to 2007. The geographical information system and multivariate analysis were coupled to characterize the spatiotemporal varia -tion of the runoff and identify its influencing factor. The results show that the hydrological condition in the Jiulongjiang RiverBasin was altered significantly in the past 40 year, represented by a decreasing tend for FI and an increasing trend for BFI.Furthermore, the watershed area is the major natural factor leading to spatiotemporal variability of the runoff flashiness index for thenorth reach and west reach of the Jiulongjiang River. The Dam construction and hydropower development made the hydrologicalcondition changed in the Jiulongjiang River Basin, which greatly influenced the seasonal variability of the runoff and sediment discharge. The land use change was identified as a potential factor influencing the runoff variation of the Jiulong River Basin.