Analyses and computations of the natural frequencies of external prestressed concrete structures are the bases for studying the dynamic properties of such structures.We carried out dynamic tests on three types of external simply supported beams,pulling the beams gradually.Then vertical loads were applied to the beams while the frequencies and related coefficients were measured at every step.We calculated natural frequencies and the results indicate that natural frequencies increase as the prestressing force rises in the pre-crack stage,and decrease significantly during the post-crack stage.Substantial incoincidences exist between the calculated and experimental results for the frequency and its tendency to change.Based on the experimental results,we modified the stiffness and other parameters in the equations.The results calculated using the modified equations agree with experimental results well,so the modified equations can be used practically. Analyses and computations of the natural frequencies of external prestressed concrete structures are the bases for studying the dynamic properties of such structures. We carry out dynamic tests on three types of external simply supported beams, pulling the beams gradually. Chen vertical loads were applied to the beams while the frequencies and related coefficients were measured at every step. We were assigned natural frequencies and the results that natural frequency increase as the prestressing force rises in the pre-crack stage, and decreased significantly during the post-crack stage. Substantial incoincidences exist between the calculated and experimental results for the frequency and its tendency to change. Based on the experimental results, we modified the stiffness and other parameters in the equations. The results calculated using the modified equations agree with experimental results well, so the modified equations can be used practically