In this paper, the free vibration of a three-layered piezoelectric semiconductor (PS) rectangular beam is studied within the framework of the theory of Timoshenko beam and the theory of piezoelectric semiconductor. The perturbations of displacement, electric potential and electron concentration are given according to the theory of Timoshenko beam. By introducing the assumption of plane stress, the governing equations are integrated along the beam thickness direction, and the governing equations including motion equation, Gauss law and current continuity condition are obtained. The vibration of a simply supported three-layered piezoelectric semiconductor rectangular beam is realized by solving the governing equations. The effects of the ratio of layer thickness, length to thickness ratio, width to thickness ratio and length on the vibration of three-layered piezoelectric semiconductor rectangular beam are shown by numerical examples. The results show that there is an initial electron concentration range where the natural frequency of PS beam decreases sharply with the increase of the initial electron concentration. The damping characteristic of PS beams increases with the increase of the layer thickness ratio. For different length dimensions and length to thickness ratios, the damping characteristics of PS beams decrease with the increase of length and length to thickness ratio.