The frequency response of a periodic beam with a propagating disturbance was studied by using the multi reflection method. A propagating disturbance is incident upon a discontinuity and gives rise to transmitted and reflected waves. At first, the wave field of segment considering incident disturbance was set up between two discontinuities due to multiple reflections. Then, wave field was set up when wave transmission took place in two coordinate directions. The first correction terms on segment are due to the leftward propagating components on all segments to the right of the segment when only leftward propagation is permitted, and the second correction terms on segment are due to the rightward propagating components on all segments to the left of the segment. So all of the transmitted and reflected waves of given flexural wave incident upon the beam at some specified location were found and superposed. The relation between the wave field of incident waves and the wave field of resulting waves on any segments was expressed. The propagating constant and iteration times of wave field of periodic beam were determined.