Abstract:The assumption in the analytical method is not adopted, that is, the prestress increment of external tendon is proportional to the mid-span vibration displacement of the beam. The energy method to analyze the dynamic behavior of externally prestressed beam is presented. Due to not adopting the assumption in the analytical method, the natural vibration frequencies are more accurate, and conform well to the numerical results. The calculation results show: the effect of external tendon on 1st frequency is underestimated, and the effect on 2nd frequency is overestimated by the analytical method; the influence of externally prestressed tendon on 1st frequency is significant, the influence on higher frequencies is slight and can be neglected; with increasing the area and the eccentricity of external tendon, the frequency of the beam increases. Theoretically, the external prestress can reduce the 1st frequency of beam, but internal prestress has no influence on the frequency of beam.