This paper studies theoretically the dynamic characteristics of RC damage beams strengthened by prestressed CFRP sheets, and the relationship between frequency and pretension is derived. The author has test on the dynamic characteristics of RC beam strengthened by prestressed CFRP sheets; the primary frequency value is measured under different pretension. The author sets up the finite element model of good concrete beam in use of ANSYS software, and gets the reduction factor of elastic modulus of concrete by the optimization tools of ANSYS based on the analysis of damaged beam dynamic test; according to the primary frequency value of good beam strengthened by prestressed CFRP sheets, a linear fit to the formula of calculating the frequency is carried on, the author gets the factor of frequency and the formula for calculating primary frequency value of damaged beam. Lastly the theoretical value of primary frequency is compared with experimental values. At low prestress, the theoretical calculation results reflect the trend of experimental data; the results are in good agreement.