The paper firstly introduces the logical framework of the security situation in the city based on the perspective of emergency and designs the structure of fourdimensional evaluation index system of the degree of urban safety from the aspects of social safety, production safety, public health safety and environmental safety; Secondly, it proposes mutation types of the safety situation in the city based on catastrophe theory and constructs the catastrophe model to measure the degree of urban safety, then put forward assessment methods and procedures; Finally, the paper uses the model above to determine the degree of safety of Shanghai.