Abstract: Dynamic tests of five full-prestressed concrete beams were carried out, the results of which indicated that the frequencies of those beams grow in step with prestressing force. It’s opposite to the analysis of dynamic characteristic of isotropic beams subjected to axial compressive force. In this paper, considering those beams as isotropic, the stiffness of full-prestressed concrete beams was modified. Considering prestressing force as internal force of beams, the natural frequency of the beams were calculated using the formula of frequencies for simply-supported beams It indicated that the error of the first frequency calculated by the modified stiffness was small. It also can reflect the variant tendency that frequencies increase with prestressing force, and it is more suitable than other modified stiffness methods. The error of the second frequency is larger than the first frequency, but is still acceptable. Large error exists for the third and high-order frequencies, which deserves further research.