In this paper, a variety of previous coating methods are reviewed for the film coating on the inner surface of long-straight tubes. A new magnetron sputtering method is introduced for the film coating in the thin and long tubes made by the conductive and non-magnetic material. The coating process of this method is as follow: the long rod target connecting the cathode is penetrated in center of the coated tube connecting the anode; a solenoid coil with DC exciting current is set outside of the tube so the steady axial magnetic field is generated in the ring discharge area between target and the tube; the working gas flows through the tube from one end to the other, the gas discharge and the magnetron sputtering coating occures in the region corresponding to the solenoid coil; the homogeneous film is obtained on the whole inner surface of the tube when the solenoid coil moves along the tube. The principle and structure of the coating system are analyzed. The distribution of electromagnetic field and electron trajectory in the electromagnetic field are calculated within the discharge region. And distribution of the magnetic field is simulated by COMSOL multiphysicsn software.