A theoretical and experimental investigation of the vibration characteristics of fluid-filled tank with internal pressure is presented. The dynamic equations of vibration are based on the elastic mechanics and velocity potential theory. The form related to the difference of pressure inside and outside was given by analysis of its effects on the tank. Also the form related to the solid-fluid interaction was given by analysis of its effects. The classic boundary-value approach was used here to calculate the natural frequencies of the tank. A modal experiment about vibration characteristics of fluid-filled tank with internal pressure was conducted, and the frequencies were measured. Results of theoretical calculation agree with the experimental data, which indicates that the frequencies of the tank decrease when filled with liquid, but they increases with internal pressure, compared with that of the empty tank. Further analysis of the results shows the effects of factors, such as the wave number, internal pressure, fluid density, structural geometric parameters and flexural rigidity of the tank on the dynamical behaviors.