Researches of wave motion are important on theory and application. The forward problem and the backward problem are the two aspects that relate with each other. In this paper, with the combination of FEM considering BGT absorbing boundary condition for forward problem and GA method, the inversion of target’s characters in infinite area is discussed. The absorbing boundary condition is obtained through approximating the Sommerfeld radiation condition. Although the absorbing boundary condition can’t simulate the effect exactly, it can reach the limitation in the practice and has the character of solving the coupling that simplifies the calculations. Because of the use of finite element model method, the forward problems that involve special geometrical shaped scatter and multi scatters can be solved. The finite element model is used iteratively to compute the scattered field. In the inversion process, the GA method is adopted because of its advantage of global optimum search and parallel calculation. The numerical simulation examples of single and multiple show the validity of this method.