Analysing requirements is a major challenge in the area of safety-critical software, where requirements quality is an important issue to build a dependable critical system. Most of the time, any project fails due to lack of understanding of user needs, missing functional and non-functional system requirements, inadequate methods and tools, and inconsistent system specification. This often results from the poor quality of system requirements. Based on our experience and knowledge, an environment model has been recognized to be a promising approach to support requirements engineering to validate a system specification. It is crucial to get an approval and feedback in early stage of system development to ensure completeness and correctness of requirements specification. In this paper, we propose a method for analysing system requirements using a closed-loop modelling technique. A closed-loop model is an integration of system model and environment model, where both the system and environment models are formalized using formal techniques. Formal verification of the closed-loop model helps to identify missing system requirements or new emergent behaviours, which are not covered earlier during the requirements elicitation process. Moreover, an environment model assists in the construction, clarification, and validation of the given system requirements.