Fixed fire extinguishing systems are installed on civil aircraft to extinguish fires in areas inaccessible to the crew, such as the engine compartment, APU (Auxiliary Power Unit) compartment, cargo, etc. For engine compartment fire extinguishing, actual flight tests in China were usually conducted to show the concentration of fire extinguishing agents sprayed under critical air flow conditions meets airworthiness requirements. Some researchers analyzed the factors affecting the concentration distribution of fire extinguishing agents in the power plant compartment, as well as the low-temperature release method. This article combines the flight test experience of a certain domestic civil aircraft model to introduce the flight test method design, test modification content, flight test implementation, and risk assessment of the fire extinguishing agent concentration test subject, which can provide guidance for the test flight of fire extinguishing agent concentration testing subjects and also provide reference for other aircraft models to carry out related test flights.