Six hypotheses were drawn based on machine middle ear model, and mechanical model was constructed. Functional relationship of displacement and sound pressure was got by variational principles and Bessel function. Then analytic solution was obtained, and moreover ear ossicle displacement was obtained. Comparing the results with the test , the results show the correctness of equation. The equation of this paper can be applied on estimating the acoustic characteristics of ossicular well and avoiding disturbance of condition and technical level of personnel. Furthermore this method is easy and this equation can be used to optimize the material and shape of ossicle.