The invention discloses a machining device for machining a polygon and a machining method thereof. The machining device comprises a main box body and an auxiliary box body which are arranged side by side, a first shaft is installed on the first main box, a cutter head is arranged at one end, close to the auxiliary box body, of the first shaft, the cutter head is provided with a cutting tool, and the end, where the cutter head is arranged, of the first shaft is further provided with a spline groove sleeve; a second shaft is installed on the auxiliary box body, a spline groove is formed in one end, close to the main box body, of the second shaft, the spline groove is matched with the spline groove sleeve in installation, a driving wheel is installed on a position, located inside the auxiliary box body, of the second shaft, and the common multiple of the ratio of the number of teeth of the driving wheel to the number of teeth of a driven wheel is equal to the number of sides of the polygon to be machined; the driven wheel transmits power to a driven wheel installed on a third shaft through a middle idle gear, and a chuck is installed at the end, close to the main box body, of the third shaft. The machining device can achieve one-time machining forming for polygon components, and compared with working efficiency of an existing milling machine and a planning machine, working efficiency of the machining device can be increased by 2-3 times.