The microscopic processes of internal force redistribution of structural system due to dead load failure or fatigue failure of structural element and failure mechanism of cumulative damage under outside loads is discussed in this paper. Considering the influence of cumulative damage to the residual strength and the reasonable time of dead load failure selected, the next step calculation of cumulative damage has a clear quantitative time. Reasonable expression is provided for the establishment of the equation of safety margin. By an example, the reliability index, which is gained by rough conservative considering the time of dead load failure selected, is compared with the reliability index, which is gained by using the method of this paper. The method of this paper coincides with the engineering fact by analysis. A reasonable method is provided for reliability analysis which structural system is considered dead load and fatigue failure at the same time.