Abstract: In this paper,a mathematical rmdel for urban rainstorm water logging is established,the nund flow and river flow in the city are the main objects to be simulated.The rmdel is established on the basis of two-dimensional unsteady flow theory and the technique of nonstructural and irregular meshes.A method on the basis of one-dimensional unsteady flow theory is proposed to deal with small scale river.The different discrete forms of momentum equations are derived for different types of passages which are divided into the river type,the gmund type and the special passage type (small river in the cities),And the discharge per unit width in each passage of any grid is computed by the different discrete forms of momentum equations.The flow in the drainage pipe netcork is simulated by one-dimensional unsteady flow theory,and the calculation of the pumping stations,the water gates and submersed pipes is indicated.According to the idea of the nonstructural and irregular meshes,the topographies are considered and the calculation domains that imvlve any polygon are designed respectively in Tianjin City,Nanjing City and Nanchang City.The application and ernr analysis of the mathematical rmdel in Tianjin,Nanjing and Nanchang are showed,too.