In this thesis, we analyze and design the piezoelectric device by finite element simulation. The modeling and simulation have been applied in three subjects: admittance of piezoelectric device, directivity of distance sensor, and high intensity focused ultrasound( HIFU). For admittance of piezoelectric device, a methodology to model mechanical losses of piezoelectric devices by the finite element analysis is presented. Complex parts of the material constants are extracted using an iterative method. Mechanical losses of piezoelectric devices are taken into account through these complex constants. A scheme using the QR factorization is developed to decompose mechanical losses of an arbitrary-shaped device into fundamental modes. These losses are then transformed into an equivalent viscous damping ratio in a standard finite element dynamics analysis. The proposed method enables us to obtain a reliable mechanical loss factor. Numerical results demonstrate that the proposed method can predict measured admittance spectra reasonably well. For directivity of distance sensor, we analyze the effect of different boundary conditions to the half decay angle. The goal for vehicle short-range sensing is that the half decay angle in the vertical direction has to be less than that in horizontal. Our simulation reveals that the half decay angle of free end boundary condition is much less than that of simple-supported and fixed ends. With such design criterion in mind, we further improve the distance sensor device by cutting a square hole in the vertical direction to mimic the free-end boundary condition. This procedure can improve the asymmetry of directivity up to 50%. However, it shortens the detect region. Therefore, we change the length of cave and obtain an optimized value, 0.4 times of device diameter, for the directivity and detect region. The HIFU application requires the length to width ratio of -3dB focal area to reach a maximum value. To this end, we construct the HIFU simulation model to find out the relations between the modal shape and resonance frequency, and compare the numerical result with experimental measurements. Furthermore, we change geometric focal length and angle in the same active area to explore the effect of -3dB focal area. The result reveals that small geometric focal length and large angle may decrease the -3dB focal area, and increase length to width ratio and the pressure magnitude of main lobe. In conclusion, we have successfully shown the feasibility of using finite element simulation to analyze the behavior of piezoelectric devices. Through the aforementioned simulation, we can predict the energy loss of piezoelectric material and provide a guideline for design of distance sensor and HIFU.