Float vent valves play an important role for civil aircraft fuel tank venting. During the validation of the float vent valve for a large civil airliner, there were several failures, such as freezing in the icing test and being closed in the rated flow test occur, which cannot guarantee the normal venting of the fuel tank. For the icing test failure, the structure of the valve seat, the bracket and other parts of the product was optimized, and the hydrophobic coating is panted on the surface of the valve seat. The improved product passed the test verification. For the ventilation closing failure at rated flow, fluent was used to simulate the flow field during the ventilation process of the fuel tank, and pneumatic closing torque of different flow point was extracted for analysis and comparison with the gravity torque. According to the calculation results, the product was improved and the ventilation closing problem has been solved.