The finite element of a prestressed concrete (PSC) beam is analyzed in a case of diverse prestressing forces. The mechanical model applied is that advanced by Clough for a concrete beam bearing axial force. The results indicate that the beam frequencies decrease with increasing prestressing force. A dynamic experiment of a nonbonded PSC beam was carried out to validate the veracity of the finite element analysis. Test beam frequencies increase with increasing prestressing force. The analysis model thus is unfit for frequency analysis of a PSC beam. The determinant factor of frequencies is analyzed, and the dynamic model is modified according to experimental results. The modified model analysis result shows that: the calculated frequencies of mode 1 agree with the test results well; the frequencies of mode 2 posses some errors; and, the modified model can reflect the direction of frequency change along with the change of prestressing force well.