This paper describes the preparing methods of piezoelectric AlN thin films used for FBAR devices. The piezoelectric AlN thin films were prepared using MF(40 kHz) magnetron reactive sputtering process with dual Sgun structure. The Mo electrode thin films were fabricated using DC magnetron sputtering Mo target with dual Sgun structure. The AlN thin films and Mo thin films are analysed by Xray diffraction(XRD), the results indicate the preferred orientations for the (002) crystal face of AlN thin films and the (110) crystal face of Mo thin films are excellent. The thickness measurement of AlN thin films on 4″Si wafers is carried out, the results indicate the thickness uniformity of AlN thin films is less than ±0.5%.The stress of AlN thin films and Mo thin films on 4″ Si wafers is tested, the results indicate the stress is at the range of -100 MPa to +100 MPa and -150 MPa to +220 MPa,respectively. The stress of the composite Mo/AlN/Mo/AlN film on 4″ Si wafers is tested, the results indicate the stress is -71.518 5 MPa.The composition of AlN thin films on 4″ Si wafers is tested, the results indicate the atomic ratio of this film is 51.8∶48.2.