Abstract: The dynamic test of two full-prestressed concrete beams was carried out. The test results indicate that frequencies of those beams grow in step with the prestressing force. It’s opposite to the analysis of dynamic characteristic of isotropic material beams subjected to axial compressive force. There are many factors for this result. This paper discusses the change of the elastic modulus results in the stiffness change, which then changes the frequencies. A modified formula was proposed, and the comparison of calculation results was taken. The results indicate that the method of this paper has a good applicability, the experimental results agree well with the theoretical analysis.