The system design and development of civil aircraft are often driven by requirement. In response to the problems of unclear methods and difficulty in establishing complete logical relationships between requirements in the requirements analysis process of civil aircraft systems, this paper proposes a function-based requirements analysis method (RAAD). Firstly, the concept and purpose of RAAD are introduced, and then the typical process of RAAD is elaborated in detail. Taking the comprehensive modular avionics system of civil aircraft as an example, the research and practice of requirement decomposition analysis method are carried out to sort out and analyze the typical types of functional and performance indicators of the system. Finally, summarize and evaluate the research methods, practical results, and practical outcomes. The results indicate that RAAD can better clarify the specific requirements of the system, establish clear logical relationships between upper and lower level requirements, and facilitate more comprehensive requirement development, confirmation, and verification work for complex systems.