Abstract: Finite element method (FEM) is normally used to study guided wave propagating in complex structure and the scattering field in defects. A key problem in FEM is to set the absorbing region to reduce the reflection from boundary. A method by changing the damping coefficient of material to achieve absorbing boundary in a 3D model is realized, and it is also discussed how to choose parameters such as layer number and damping.Results show that the size of the absorbing boundary is the main influencing factor, and the length should be longer than two wavelengths. To prove its validity, this model was applied to calculating scattering matrix of defects.