In order to correctly evaluate urban comprehensive disaster-carrying capability and then to establish scientific basis for disaster prevention and reduction, a fractal method for urban comprehensive disaster-carrying capability is proposed on the basis of system principle which is that structure determines function. The fractal dimensions of 29 cities are calculated and analyzed. The results show that relative stronger, middling and weaker are respectively 13.79%, 65.52% and 20.69%, which are completely consistent with the actual situation. So the method can construct a characteristic index, and the main impact effects can be analyzed according to the result. Tested by examples together with the other two methods, i.e. the fuzzy assessment method and exponent method, the results verify effectively that this method is useful and feasible.