This paper aims to simulate the sound field of rectangular small enclosure excited by panel loudspeaker with FEM. Firstly, in order to testify the fluid-structure interaction theory in the field of acoustics calculation, coupled modal and uncoupled modal of loudspeaker and air were established. Result of harmonic response analysis shows that the vibration amplitude of uncoupled case is larger than the coupled case. The result just is in harmony with the fact. So when modeling the sound field, fluid-structure coupling should be taken into account. Secondly, for more accurate sound field simulation, different from most researchers’ modeling method that supposing the vibrating plate’s boundary of the loudspeaker is simply supported, this paper provide a more practical way that the boundary is elastic supported. The comparison shows that the resonance of air in enclosure is more severe under elastic boundary on condition of same driving force. Thereby the sound pressure is higher.