A method for simulating a complex system including a plurality of constituents is provided. The method includes constructing at least one complex system model, each system model including a hierarchised set of modelled constituents. The construction step includes, for each model, obtaining a multiple instance hierarchical model including at least one instance vector corresponding to a plurality of instances of one and the same modelled constituent, each instance vector being able to be located at any level of a hierarchical decomposition tree of the multiple instance hierarchical model. The construction step additionally includes, for each model, expanding the multiple instance hierarchical model into an expanded model by expansion of at least one instance vector included in the multiple instance hierarchical model.