Auxiliary power unit is an important part which is installed on the civil aircraft. It is necessary to consider the security of aircraft while APU rotor bursts. This paper simulates the impact of small fragment onto the APU-firewall, and calculates three different thickness of reinforce-plate with finite element software. According to the result of numerical calculation, an appropriate reinforce-plate thickness was chosen, which can prevent the small fragment, while the aircraft weight need increase small. By reinforcing the impact zone of APU-firewall,the security of the aircraft is ensured while APU rotor bursts, and the design requirements for AC20-128A is met.