The purpose of the article is to regard city as the object and work out a set of methods of assessment of risk. The improved technology of assessment of risk for urban major hazards and the method of quantization are studied. Principle of “level-superposed” of the grade of security is brought forward. Analyzing the possibility of all kinds of calamities to take place, confirming the grade of security relative to city of the kind calamity, and superposing its coefficient correlation in certain proportion, this paper gets the synthetical security atlas similar to the real one based on urban geography and urban resource. The evaluation index of social risk is set up based on the distributed comprehensive density of population, thus comprehensive safe planning to the dangerous source in the city can be carried on. At last, an example for providing the visual safe grade of city and the divided electronic map is given. The exploration work of this text can offer reference for relevant research work, which is also significant to improve the engineering level of urban assessment of risk, to prevent and control the emergence of great malignants accidents effectively such as fire, explosion, poisonous substance leakage, etc. , and to offer the scientific decision basis for city safety management.