Abstract: Auxiliary power unit (APU) refers to a small auxiliary power unit that can independently output compressed air or supply power except the main power unit (engine) on the aircraft. It is generally a small gas turbine engine. When performing penetration inspection of its parts, during the dropping process, the parts are suspended and exposed to low temperature,which will increase the viscosity of the liquid and affect the penetration and exudation of the penetrant from the defect,and then affect the penetration inspection result. This article makes a simple analysis of this problem, and puts forward a simple solution to solve the problem.