Abstract: Land use change has been identified as an important contributor of runoff changes. In this study, a new model, XAJ-CN, was developed by integrating the Xinanjiang model and Curve Number of SCS-CN (Soil Conservation Service) model to assess runoff changes in the Dongjiang River basin. Three sub-basins, located in the middle and lower Dongjiang River basin, were selected as case studies, which constitute the most important water source system for Guangdong province and Hong Kong. Based on the land use and soil properties, the proposed XAJ-CN model was calibrated and validated using 10 years of data. It was observed that the simulated runoff series matched the observed one well, indicating that the performance of the XAJ-CN model was satisfactory. The proposed model was then applied to simulate the effect of land use changes on runoff variations under six specific scenarios. In the end, the impact of urbanization on environmental flow was analyzed using the IHA (indicators of hydrologic alteration) and HMA (histogram matching approach) methods. Results showed that the impact of land use change on runoff was more obvious in the flood season when compared to the dry season, and the effect of change in the CN value on surface runoff was the greatest in the flood season, while the change in the CN value mainly affected groundwater runoff in the dry season. Results also showed that the magnitude of monthly river flow all increased, and the maximum flow magnitudes of 1-day, 3-day, 7-day, 30-day and 90-day periods were larger under the urbanization scenario than those in the baseline period. In addition, there was a great variation in the frequency and duration of high and low pulses.