The reflected coefficient of functionally graded layer to waves is very important as the design of impact-resistant FGMs. In the paper, the Optimization problem of propagation characteristic of elastic wave in FGM has been investigated. The relation between the top layer's stress and displacement and the bottom layer's is deduced from elastic propagation equation as the incident wave is a plane and harmonic elastic stress wave, then the reflected coefficient of gradient layer is obtained, and the reflected coefficient is selected as object function of optimization. The thickness of each layer is selected as optimization design parameter. The conjugate gradient method is used to get the optimal results about gradient layers' thickness.