A SWMM (Storm water management model)-based method for the adjustment of rain-off model parameters is proposed to realize the equivalent design conditions in pipe network design calibration with hydraulic model. The method proceeds from the basic principle of Rational Method, and takes the assumption of runoff curves with time as the basis. By simulating the runoff catchment through constant rainfall pattern, the principle of equivalent can be judged by whether the features of the simulated runoff curves matched those of the assumption with Rational Method. Case study has verified the feasibility of the proposed method. Meanwhile, to simplify the manual operation in parameters adjustment, an improved particle swarm optimization (PSO) has been put forward to complete the process automatically.