In recent decades, the rapid development of wireless communication technologies has led to the fast development of other fields, and radio frequency (RF) front-end devices become of particular importance for the fifth-generation (5G) telecommunication system. In this context, the film bulk acoustic resonator (FBAR), with an operating frequency up to 10 GHz and miniaturized size, is therefore becoming in great demand to satisfy the emerging research and industry needs. In this work, the design of a tunable FBAR is performed and analysed by finite element method (FEM) at microwave frequency range. The FBAR tunability is realised by applying different mechanical loads and its equivalent circuit was studied by using a Butterworth-Van Dyke (BVD) model, this work gives a deeper understanding and analysis on the physical origin of FBAR's tunability and can therefore give some strategy and methodology support for the design of such devices.