In order to study the analytic expression of the dynamic characteristics with the continuous beam with external tendons, established the analysis model with any form of prestressed tendon layout based on the Bernoulli-Euler beam theory. Based on the formula presented by Ayaho Miyamoto, through established the relationship between prestressing force and displacement and taking the continuous beam as the multiple single beams satisfied with the moment and angle conditions, and derived the vibration equation of beam using piecewise simultaneous method and got the frequency equation. Taking two-span and three-span continuous beams with external tendons as an example, to solve the frequency equation and get natural frequency. Compared the results of the formulas with FEA and existing tests results, It is found the result calculated by the formulas is in good agreement with the results of FEM and existing tests. This method can get the dynamic parameters of continuous beam with external tendons accurately.