Double wall pipe ventilation system is an important system to ensure the safety of gas supply for LNG powered ships. The system can detect the leakage in time and dilute the leaked gas rapidly in case of gas leakage. The failure mode, effects and criticality analysis (FMECA) method is used to assess the risk of the double wall pipe ventilation system, identify the failure mode and cause of each element, and detect and analyze the existing safety barriers and failure effects, as well as evaluate the failure frequency, consequence severity and detection difficulty. The risk priority is then calculated for the risk classification and the design optimization is carried out for high-risk failures. New safety barriers are finally installed to reduce the risk. A total of 13 failures are identified from the design scheme of the original double wall pipe ventilation system, of which 5 were high risk. The risk level is reduced to an acceptable range through optimal design such as arranging the air flow switch in the air inlet duct, setting up independent fan rooms and dual power supply for the fan.