Abstract: A simply-supported prestressed beam is studied.The change of the prestress force in the lateral vibration process of the beam is discussed and the differential equation of lateral vibrations for the simply-supported prestressed beam is given.The mode perturbation method is used to establish an approximate analysis technique for the modal characteristics of the prestressed beam in lateral vibrations.As a result,the solution of the complicated differential equation with variable coefficients is obtained approximately by solving the algebraic equations and the computational effort is reduced effectively.The influence of the prestress force on the natural frequencies of the beam is also discussed.Numerical results show that the effect of the prestress force on the first modal frequency is significant when the eccentricity of the prestress force is large.