Aiming at the difficulty of building testability model directly for large complex system, a correlation matrix merging algorithm is proposed. Firstly, the principle of hierarchical module division of large complex system is analyzed, and the modeling elements of hierarchical modular testability model are briefly described. The merging algorithm divides the hierarchical modules of the system, and then builds testability models for the low-level unit modules. The fault-test correlation matrix of the low-level unit module is gradually merged to generate the fault test correlation matrix of the whole system. Finally, the navigation system of a certain missile is taken as an example to verify, which simplifies the difficulty of system level testability model construction.