Abstract: With depth-first left-most ergonic algorithm, the complex dynamic fault tree can be decomposed into several ISFT (individual sub fault tree) in availability analysis of complex system. In order to calculate its availability in time-variable filed, the equivalent time-variable availability parameters are presented and proved to ISFT. Transferring ISFT into corresponding Markov chain, the differential equation of sub-Markov chain in falling dimension can be calculated with time-variable availability parameters easily. From bottom to up, the availability analysis for complex system can be realized with recursion algorithm, which not only solve the problem that the computing enhances exponential with the number of states increases in availability calculation of complex system but also improve the calculation precision with time-variable parameters. Application indicates that the equivalent time-variable availability analysis based on ISFT is content under the effective decomposition.