Building density and arrangement are important factors of the runoff process in urban resident area. Based on the two-dimensional shallow water equations and the building-hole method which was used to simulate the buildings, a rainfall run off model of urban community was established. Related artificial rainfall experimental results were used to validate the model parameters, and then the influences of building density and two arrangement types of inline layout and staggered layout in runoff process were studied. The results show that the time of concentration increases with the increasing building density, they satisfy the exporential function relation. For the same building density, the time of concentration in staggered layout is larger than that in inline layout. The difference between these two layouts is increasing as the building density increases. At the cross-sections where building exists, average water depth and average flow velocity are greater than non-building situation, and they increase with the building density. At the non-building cross-sections, the average depth is similar to the situation of non-building but the average velocity is lower and it decreases with the increasing building density. The resistance coefficient increases with the building density. When the building density is less than 16%, the resistance coefficient increases slowly, but it increases significantly when the building density surpasses 16%.