1. A method for analyzing structure safety, comprising the steps of: (a) installing a plurality of vibration sensors to a testing structure; (b) using the plurality of vibration sensors to perform an initial vibration measurement to the testing structure and create basic information; (c) capturing a continuous time-domain vibration signal of the testing structure, and converting the continuous time-domain vibration signal into a frequency-domain vibration signal by a fast Fourier transform, and then reducing Gibbs phenomenon induced in the frequency-domain vibration signal by applying a window function to the frequency domain vibration signal; (d) filtering the frequency-domain vibration signal to determine an analyzing frequency range by using a filter; (e1) obtaining top three characteristic frequencies from the analyzing frequency range; (e2) graphing a moving frequency spectrum plot of the top three characteristic frequencies, wherein Y-coordinate of the moving frequency spectrum plot is the Frequency and X-coordinate of the moving frequency spectrum plot is Time; and (e3) evaluating a structural safety of the testing structure by observing the moving frequency spectrum plot and determining whether the variations exceed a critical value.