With the wide application of new materials and processes in aircraft, the possibility of aircraft fire is greatly increased. Therefore, fire prevention design is a key research direction of civil aircraft. Fire prevention measures for flammable liquids in aircraft include isolation, ventilation, liquid drainage, fire detection, etc. Among them, the drainage design of flammable liquid in aircraft is a main means of fire prevention design. Flammable liquid in aircraft is widely distributed in fuel tanks, hydraulic fuel pipelines and related system equipment which makes the fire protection design feature elements ( ignition source, flammable object, etc ) complex and numerous. During the iterations of aircraft design, due to the need to manage and control the configuration of these characteristic elements, the traditional method for the drainage design of flammable liquid will bring a lot of workload, and a new approach is urgently needed to solve it. This paper presents a liquid drainage method based on digital mock-up, and analyzes the key idea of the establishment and analysis of digital mock-up. At the same time, the drainage design and verification method of flammable liquid based on the digital mock-up was studied combined with system engineering technology.