The invention discloses a fine machining method of an inner bore of a long and thin barrel part. In the method, the stepped sizes of two ends of the inner bore of the long and thin barrel part are respectively bored finely on a horizontal lathe by utilizing a floating boring cutter, and a long cutter bar drives the floating boring cutter embedded in a rectangular hole in the middle of the long cutter bar to rotate in the fine boring process, thereby realizing the purpose of finely machining the inner bore of the long and thin barrel part. The method comprises the steps of clamping, rough boring, cutter setting and fine boring. The problem that the size precision is difficult to ensure due to cutter test many times for machining the same size in the traditional machining process is solved by adopting the machining process for finely boring the inner bore of the long and thin barrel part on the horizontal lathe by utilizing the floating boring cutter, and the problems that the size of the inner bore is difficult to precisely measure and the coaxiality is difficult to ensure in the machining process of the long and thin barrel part are solved, thus the machining quality of the long and thin barrel part is effectively improved.