In recent years, with the rapid development of urbanization in Wuhan and the background of global climate change, urban water logging has occurred from time to time. To some extent, it has affected people's production and life. The survey found that there is a direct relationship between urban water logging and the backwardness of drainage system. Taking Hannan District as an example, the size of the implemented four-way node reserved box culvert under a municipal road is only 2 holes and 5m×4m, which has become the bottleneck of the new porous large section rainwater box culvert in the upstream, so it is urgent to expand and transform. Taking this section as an example, this paper analyzes the capacity expansion of box culvert overflow section to meet the requirements of section overflow.