Abstract: A new approximate algorithm for calculation of the failure probability of a system is presented. Firstly, an equivalent linear failure mode is introduced to approximate the conditional failure mode; Secondly, a new approximate algorithm for calculating the system reliability of parallel structures is obtained based on the principle of conditional probability and the theoretical expression of conditional failure mode. Thus, a complicated calculation of failure probability of a group of failure modes'intersection is simplified to the calculation of probability product of a group of equivalent linear failure modes. Because the serial system and the mixed system can be expressed as a linear superposition of a series of failure modes?intersection, the algorithm is suitable to the calculation of failure probability of the series and mixed systems. This algorithm is simple, effective and accurate. It is also adaptable to the reliability analysis of large structure systems.