In order to research the influence of the flow field around the large-scale flexible airship on its modal performance, the air around the airship was assumed as compressible potential fluid, the influence of air with structure vibration is considered as additional mass was added to the capsule structure. The modal test of the internal pressure of the 1:60 main airbag at 2kpa、4kpa、6kpa、8kpa and 10kpa was completed, the virbrometer based on the principle of laser vibration measurement was applied to measure the first three modes of the main airbag. Also the wet modal analysis method based on the additional mass was used to analyze the first 3 modes of the main airbag under 5 different internal pressure. The test and calculation results prove that the wet modal analysis method on the additional mass can be used for modal analysis for large flexible structure.