According to the characteristics of the triangular plate element with fine boundary adaptability, simple interpolation displacement function, and low freedom degrees, the area coordinate of the element is adapted to obtain the displacement interpolation function of the plate, so that the dynamic modal of the plate is achieved and analyzed. Then, in order to discuss the vibro-acoustic problem of the three-dimensional elastic cavity with high precision, the triangular plate element is used to discretize the elastic plate domain, whereas the smooth finite element method with high accuracy is applied in the acoustic cavity domain. Considering the coupling interaction between the plate and acoustic cavity, a new model for the three-dimensional elastic cavity is obtained, and the vibro-acoustic characteristics for it is demonstrated. Numerical results with comparison show that the calculation scope is smaller with accurate precision. Furthermore, it can be applied to the analysis on the vibro-acoustic characteristics of more complex three-dimensional elastic cavities.