Abstract: Two numerical methods for analyzing dynamic behavior of externally prestressed beam are put forward: between external tendon and deviator, slip without no friction is assumed in method ①, and is simulated by contact element; hinge without slip is assumed in method ②. The calculation results of method ① show: the increment of area and eccentricity of external tendon increase the frequency of the first order; influence little the frequencies of other odd order, don’t influence the frequencies of even order. The frequency of every order by method ② is greater than the one by method ①. With increasing area and eccentricity of external tendon, the frequency of every order by method ② increases; the ratio of frequency difference between method ① and ② also increases. The influence of slip between external tendon and deviator to the frequency of the first order can be neglected, but this influence to the frequency of other order can not be neglected.