The air-conditioner for railway vehicle should have vibration durability in order to operate normally in vehicle running situation. KS R 9144(vibration test methods for railway vehicle parts) is used to verify the vibration durability. The specifications of compressor, condenser and evaporator for air-conditioner in railway vehicle are standardized, but the shape and structure of pipe lines are not specified. Because the air-conditioner handler produces the pipe line arbitrarily, sometimes the pipe line is broken during the vibration durability test. In this research the cause identification and solution of pipe line breaking problem in during the vibration durability test were studied for air-conditioner of railway vehicle(diesel multiple unit). It was found that the natural frequency of pipe line is related to the pipe line breaking by experiment. A new pipe line shape was introduced by using FEA in order to avoid the resonance. The prototype new pipe line was applied to air-conditioner and the natural frequency was measured by experiment in order to verify the resonance avoidance. The vibration reduction of air-conditioner with new pipe line was reviewed by comparing to the air-conditioner with original one.