Taking prestressed simply supported beam as an example, the change of prestressed beam in the process of transverse vibration is analyzed, and the differential equation of transverse bending vibration of prestressed beam is established. The modal perturbation method is used to further derive the approximate analysis method of the modal characteristics of prestressed beams, and the solution of the complex variable coefficient differential equation is transformed into the solution of linear algebraic equations, thus simplifying the calculation process effectively. Finally, the influence of prestress on the transverse vibration characteristics of the beam is discussed by an example. The results show that when the prestressed position has a large eccentricity, the prestressed position has a great influence on the natural vibration characteristics of the beam.