Abstract: The rapid development of urbanization has led to a large number of urban buildings and increased density. Dense buildings make the urban flood situation more complicated and bring rigorous challenges to urban flood simulation. The urban hydrological model is a key means of urban flood research. an urban pipe network and 2-D surface coupled model are constructed by flood analysis software Integrated Flood Modeling System（IFMS） in Liedechong watershed, Tianhe District, Guangzhou, and it is validated with experimental data. Then the high resolution digital surface model（DSM） is used to perform elevation interpolation on the grids with resolutions10, 20, 30 and 50 m, respectively. After the best grid resolution has been selected and buildings have been selected as the research object, the high resolution digital elevation model（DEM） and DSM are used to interpolate the grid elevation, and the situation of urban flood under dense buildings is explored by comparing the surface buildings depicted with DEM. The results show that when the grid resolution increases from 10 to 50 m, the inundation water depth become greater and the inundation range become wider, make the flood situation more serious. In the case of dense buildings, the proportion of inundation area increaseS from 15.46% decreases to 6.67%, and the maximum inundation water depth increases from 0.20 to 1.78 m. The conclusions are as follows: In the urban flood simulation research on buildings, the grid resolution should consider the actual building width, and the best simulation effect is when the grid resolution is 10 m; dense buildings reduce the inundation area and sped up concentration. The collected rainwater makes the drainage pipe network insufficient carrying capacity, resulting in overflows to form surface water and increase the depth of flood submerged water.Compared to interpolated with DEM, the use of DSM interpolated elevation can more truly reflect the current urban surface characteristics and improve simulation accuracy. The research conclusion can provide a scientific basis for urban construction and development.