This paper introduces a method of modelling multistate repairable multi-unit systems' behaviours subjected to stochastic degradation processes, using the coloured Petri nets (CPN). This solution prevents the combinatorial explosion problem that reduces the use of Markov graphs for studying such systems. The method combines the ability to model the state and evaluate the operational performances of a complex system. The method also simplifies the study of k-out-n structure systems, while allowing extraction of the minimal cuts. The method provided is grounded on theorems used to design an algorithm of systematic construction of models.