The membrane structure has good application prospects in large spacecraft membrane antenna， solar sail and baffle because it has the advantages of low quality and small folding volume. The modal characteristics of membrane structure directly decide or to large extent affect the surface accuracy maintenance and vibration control of the structure. The correctness of modal analysis needs to be verified by ground tests. Because the membrane structure is light and flexible， air has an important influence on its modal analysis results， so it is very necessary to carry out modal tests in vacuum environment. In this paper， taking the planar membrane antenna structure as a research object， a set of modal testing system is designed and built， which is suitable for the vacuum environment. The modal test under various working conditions is completed， and the effective test data are obtained. Through data processing and comparative analysis， not only the effectiveness and feasibility of the testing system is verified， but also the specific influence of air on the modal test results is analyzed. As a result， some useful conclusions are obtained. This experimental research lays a solid technical foundation for the modal characteristics investigation and design improvement of the membrane antenna， which is of great engineering significance.