Abstract: Aiming at the complex boundary in the numerical calculation of flood diversion area,a bevel angle Cartesian method is adopted based on the traditional Cartesian method.The bevel angle Cartesian method has the merits of the traditional Cartesian method and can fit the actual water boundaries better.The flood routing in the flood diversion area of Baitan Lake,in Hubei Province,is calculated by the developed plane 2-D water mathematical model for flood diversion area.