Finite element analysis (FEA) is widely used to optimize the acceleration sensitivity of the optical cavities in Pound- Drever- Hall frequency locking method. In this process, mounting structure of the cavity is usually modeled by directly fixing small areas on the cavity or by using some rigid supports. The analysis is extended to more practical situations where the cavity is rest on some soft supporting materials. When subjected to forces, the cavity supported by soft materials exhibits a relatively large global rotation, interfering with the calculation of the length change. A method of coordinate transformation is introduced to deduct the effect caused by the global rotation. With this procedure added, the acceleration sensitivity of a rectangularbar cavity supported by soft materials is analyzed and the results are in some differences with the results obtained by directly fixing the cavity at the same locations. The choice of the the friction coefficient and the modeling of the supporting structure in the case of using a soft material to support the cavity is also discussed.