Urban flood is occurring frequently because of recent local torrential rainfalls, climate change and the problem of urbanization. In many studies, hydrological changes in urban areas due to the increase of impervious areas are the main causes of urban flood. However, there are insufficient studies to address the problem of population growth, which is the root cause of the increase in impervious areas by urbanization. So, in this paper, we focus on population growth in the urban flood problem and investigate the effect of land use on the urban flood from the increase in population. As a methodology, we used a system dynamics approach using a feedback method with causal loop, which is suitable for modeling natural phenomena such as population growth and urbanization. Through this methodology, we have developed a system dynamics model that calculates urban flood risk. The urban flood risk varies with population growth, urban expansion, and land use. In addition, the scenario was constructed based on the population and land use data of Yangsan City, a new city in Korea. The sensitivity was analyzed through simulation. Based on the results of the sensitivity analysis, we propose a land use regulation direction to reduce the risk of urban flood in new cities that are undergoing urbanization by population growth.