Abstract: With the hierarchical decomposition technology, the dynamic fault tree of complex system can be simplified into individual modular, which assumes that there is no intrinsic relation with other modulars. The equivalent time-variable availability parameters of individual modular such as equivalent failure rate and equivalent repair rate were deduced, the system availability can be obtained through recursive algorithm with equivalent availability parameters from bottom to up. Actually, the hierarchical availability analysis method exist some errors comparing with availability analysis based on whole states due to the hierarchical decomposition. The cause of error formation because of failure dependence and repair dependence was analyzed, the error definition in different situation was investigated and the error distribution field was calculated. The suitable condition and application principle were presented to improve the modeling error of hierarchical availability analysis. The application indicates that error analysis of hierarchical availability analysis is helpful to reliability analysis of complex system.