From the theory of flexural vibration of the thin circular plate, the analytic expressions of the vibration frequency and added virtual mass incremental factors for flexural vibration circular plate in fixed boundary conditions are given. The vibration frequency of thin circular plate are calculated in the air and fluid. Based on finite element method, the numerical simulation model of the bending vibration of the thin circular plate in the fluid is built. The characteristics of circular plate materials and the thin plate bending vibration in the different fluid are analysed. Simulation results show that different fluid leads to the different additional mass to the bending vibration of thin circular plate, thus will affect the frequency of the bending vibration of thin circular plate.By measuring the frequency of thin circular plate bending vibration in the air and the fluid, the density of the fluid can be obtained. Our results will provide theoretical basis for measuring the fluid density using the flexural vibration of the thin circular plate.

circular plate; added virtual mass incremental factors; flexural vibration; vibration frequency