In the history of international civil aviation, there have been many air crashes caused by engine compartment fires. In order to ensure flight safety, finding out various factors that affect the concentration of fire extinguishing agent in the engine compartment, adopting precise modification methods, and establishing effective flight testing methods is particularly important. This paper analyzes the influence of ventilation flow, flight speed, ambient temperature and other factors on the concentration of fire extinguishing agent in the engine compartment. It gives the test equipment, sampling pipe laying, sampling point layout and other modification methods. It puts forward the flight test method of the concentration test of fire extinguishing agent in the engine compartment, analyzes the margin of the test results, and provides a reference for the test of the concentration test of fire extinguishing agent in the engine compartment of other civil aircraft.