The object of research is a critical IT infrastructure. One of the most problematic places in the study of critical IT infrastructures is the complete lack of approaches, methodology and tools for designing, modeling and researching critical IT infrastructures that could be used in the form in which they are offered.On the basis of expanded open hybrid automata, an approach is proposed that will allow to compactly describe the components, critical IT infrastructure systems and their interrelations, both internal and external. Its peculiarity is the use of an extended set of parameters, which makes it possible to provide probabilistic and qualitative features to models of components and systems of a critical IT infrastructure.In the course of the research, the Matlab software package is used, which allows to check the proposed approach and models for workability.The resulting models are fairly compact and completely reflect the necessary logic of the work of the relevant components and critical IT infrastructure systems. It is shown that this is achieved due to the flexibility of the proposed mathematical apparatus, namely the possibility of creating compositions from simple models for the formation of more complex ones.In the future, the proposed approach and the creation of a library of models for all major systems and critical IT infrastructure components will provide a convenient tool for a wide range of researchers whose work is related to all aspects of researching critical IT infrastructures.