Abstract: In this paper the expressions for the unsafe probability and the operating reliability of a cascade safety system have been derived from the static and dynamic reliabilities of the individual safety devices (or switches). They are principally determined by the corresponding values of the individual safety devices. Both values decrease with the number n of the cascade safety devices, therefore, they are contradictory each other. A suitable number for n should be chosen for making appropriate compromise between them. However, safe reliability must be fulfilled with sufficient large number of n, so that operating reliability has to be achieved with some special measures such as redundancy in the release devices of the individual safety switches. From the definitions of the unsafe prebability and the operating reliability of a cascade safety system, we know that the reliability of the last released safety device is the most important factor for them. Then we obtain that the best structure for the most reliable cascade safety system is constructed with the more reliable devices arranged for the later release. In addition, the damage cost of the failure happened earlier is higher then that happened later, then the total predictive damage cost for one shot can be calculated by means of the weighting coefficients of' failures happened in each releasing step.