Dynamic tests on three pre-stressed reinforced concrete U-shaped beams,with the scale of 1:10,were carried out by using ambient excitation technique and filtering technique based on S-transform,by virtue of the basic theory of natural excitation technique (NExT) and ibrahim time domain technique (ITD).The modal parameters extracted from the tests results were used to analyze the influences of different sampling frequencies,flange damage of main girder web and different load excitation on the dynamic characteristics of U-shaped beams.