[Objective] To solve the issues in area superposition and flow superposition method in order to improve the discharge design in storm sewer.[Methods] Flow superposition method was modified by correcting the calculation method of storm intensity in downstream sewer. A maximum flow method based on exhaustion method was proposed.[Results] A case study indicated that the design discharge with improved flow superposition method was less than that of area superposition method, except for the initial sewer. The average design discharge based on maximum flow method was increased by 7.0%, and the maximum increase rate was 13.0%.[Conclusion] Maximum flow method is more reasonable and safer, and is of great importance for relieving urban flooding.