FAA published 25-106 and 25-127 amendment in 2002 and 2008,in order to enhance the safety property via adding security considerations for flight deck door. The amendment demand that the flight deck door should be enhanced. The FDD (Flight Deck Door) should be designed to resist forcible intrusion into cockpit by unauthorized persons and resist penetration by small arms fire and fragmentation devices to ensure safety for crew.FDD system can detect the decompression of cockpit and release the door panel to equal the pressure and must be provided to enable flight crewmembers to directly enter the passenger compartment from the pilot compartment if the cockpit door becomes jammed. There is a emergency access in the door. This article will discuss the function failure and identify hazard according to FDD feature of structure and function. The safety target which is defined based on the hazard assessment will assist the designing of FDD.