Aiming at the characteristics of drainage in the plain city, the paper put forward to the calculation method of drainage modulus in plain city based on water storage curve. Taking the main urban area of Suzhou as case study, it used the capacity curve method to calculate its drainage modulus. The result of water storage curve method is smaller than that of the simple method.It finally provided the relationships between water storage capacity and the area of cross-sectional and drainage. It can provide a basis for planning of city and channels.