A method for acquiring a dynamic vibration frequency includes the following steps. At least five pairs of vibration measurement elements are selected. A bandwidth range of the vibration measurement elements is determined. The vibration measurement elements in sequence on a first side and a second side, which are geometrically symmetrical to each other, of a main shaft are used to measure vibration displacements in a symmetrical position arranging manner. The vibration displacements measured by the vibration measurement elements of either of the first side and second side are corrected, so as to differentially eliminate an ambient noise. Positions of nodes located on the first side and the second side are calculated. The vibration displacements in the preceding steps are used to deduce dynamic vibration frequencies of main harmonics. A vibration mode of the main shaft is confirmed.