Aiming to the characteristics of drainage in the plain city,the paper put forward 1-D unsteady flow method by analyzing the principles and results of other methods.Taking the main city of Suzhou in Anhui province for example,the paper used three methods including the average elimination, cutting flathead law and unsteady flow method to calculate the drainage modulus.The result indicated that the unsteady flow method is more applicable in calculation of drainage modules of plain city and can provide a basis for urban waterlogging.