According to the catastrophe characteristics of the water supply systems when it damaged by emergency, a new vulnerability assessment model was proposed based on catastrophe theory. Taking the earthquake disaster as an example, firstly, according to the damage mechanism of water supply systems under earthquake and the historical earthquake data, the vulnerability assessment indicators was established. Secondly, according to the relationship among the control factors of water supply systems’ functions, the status function and control variables were determined. Thirdly, the cusp catastrophe mathematical model was established, and the normalized formula was derived and the vulnerability classification standard was determined. Then, based on the research results above, the new vulnerability assessment model of water supply systems was proposed. At last, the water supply system of north china was taking as a case study. Through application we could found that the model was theoretical and practical feasibility. It could be used to supply the decision support for the design of water supply systems.