The invention discloses a machine tool excitation method based on the cutting of a boss work piece with gradually changing width. A boss work piece with gradually changing width is cut by cutter teeth to generate a pulse force with gradually changing pulse width to excite a machine tool. The method comprises the following steps: (1) the gradually changing widths of a work piece boss are determined, including a narrow-end width anw and a large-end width abw; (2) the length al of the work piece boss is determined; (3) a boss work piece is machined according to the length and width parameters; and (4) the boss work piece is cut by cutter teeth according to the rotation speed n, the feed rate af and the back engagement ap of cutter teeth of a machine tool, the cutting force between a cutter and a gradually changing boss applies a pulse excitation signal to a machine tool structure so as to generate a pulse cutting force with gradually increasing pulse width and amplitude to excite the machine tool. The method of the invention not only can greatly reduce the excitation cost of modal test, but also can be adopted to excite a machine tool in a controlled way according to the needs and specific objects in the working state to obtain the dynamic characteristics of the machine tool in the working state.