A method is to excavate main roadways in the upper and lower parts separately and an inclined intake roadway in the central part of the mine and pre-excavate an inclined seam roadway as the first mining face at the boundary on one side of the mine. Staring from the mine boundary, the retreating mining shall be carried out on the first mining face strip by strip with belt conveyors arranged in the upper main roadway and assistant conveying devices in the lower main roadway and inclined intake roadway; the open-off cut of the first mining face shall be built on the underside of the upper main roadway at the boundary of the mine to carry out the downward inclined mining on the strike and along the inclination.