Abstract: In order to investigate the natural frequency of a prestressed composite beam, an analytical model of a prestressed steel-concrete composite continuous beam is formulated. Based on the formula presented by Ayaho Miyamoto, the formulas for natural frequency of a double broken line prestressed composite continuous beam with external tendons are derived. The all natural frequency formulas for the continuous beam is derived through Laplace transform, meanwhile, the influence rule of the composite continuous beam due to an equivalent axial force is analyzed. The results of the formulas are compared with existing tests and FEA results, and the first calculated frequency of the continuous beam is consistent with the other results. Meanwhile, the influence rule of the theoretic formulas on the composite continuous beam due to the equivalent axial force is consistent with FEA. The natural frequency of a prestressed composite beam with external tendons can be calculated by the derived formula.