In this paper, based on the Bragg scattering mechanism, the influence of material sound velocity on the band gap characteristics of twodimensional AlN/B/Si composite phononic crystal is studied by using the finite element numerical simulation technology. Firstly, the optimum thickness of each layer of AlN, Mo, Si are analyzed successively by using Mo as the B layer when the maximum band gap width of AlN, AlN/Mo and AlN/Mo/Si occurs. Then the material with sound velocity gradient is selected as Blayer to study the effect of sound velocity on the band gap width change rate of composite phononic crystals. The results show that when the difference of sound velocity between Blayer and Si and AlN is less than 3 000 m/s, the change rate of band gap width of composite phononic crystals is less than 25% due to the change of Blayer thickness. The bigger the difference of sound velocity between Blayer and Si, AlN, the bigger the change rate of band gap width will be caused by changing the thickness of Blayer, up to 100%.