With the deepening of the Belt and Road Initiative，a lot of Chinese companies undertake construction projects in Africa，such as port，highway and industrial park. With the development of the project，the drainage near the project site needs to be constantly changed and the drainage planning near the project site is particularly important. Based on the Civil 3D terrain analysis module，confluence analysis module and confluence region boundary analysis module，this paper presents a design method for analyzing the large-area drainage and planning of large-area drainage. Based on Kribi Deep Sea Port Project，this paper expounds the application of this method，which provides reference for the whole area design planning for other large projects.