Abstract: A semi-analytical approach based on the proper orthogonal decomposition (POD) method with a Galerkin projection scheme is adopted to construct a reduced order model (ROM) to solve three-dimensional panel nonlinear aeroelasticity problem in supersonic flow, using von Karman large deflection theory and piston theory. Orthogonality of the transform matrix between the numerical POD modes and harmonic basis functions of the Galerkin method is proved, which simplifies the construction of POD/ROM. The accuracy, convergence and efficiency of the POD method are verified through numerical examples. It reveals that the POD/ROM can recover the Galerkin solutions with the same accuracy and computational efficiency, using fewer modes. When considering an example with a length-to-width ratio of 4, 2 modes and 3 seconds are required in the POD method to describe panel oscillation, while 16 modes and 900 seconds are required in the Galerkin method.