The invention provides a field processing method for grinding the surface of a slip ring in a feeding slip ring body. A supporting frame is built beside the feeding slip ring body, a lathe is installed on the supporting frame, a grinding wheel is installed on a motor output shaft the rotation speed of which is 2800-3000rpm, a motor base is fastened to the bottom slide of a saddle which transversally shifts along a guide rail of the lathe, and a protective cover is assembled on the non-working side of the grinding wheel. The field processing method comprises the following steps: jacking the feeding slip ring body end of a cylinder through a high-pressure pump of a large slipper mill lubrication station; starting a low-speed motor of a driving device of a large slipper mill so that the cylinder rotates; and carrying out surface grinding processing on the position with the maximum outer diameter, of the slip ring, wherein the surface grinding processing is divided into coarse grinding and fine grinding; in coarse grinding, the roundness and straightness of the slip ring can be achieved by coarse grinding via a coarse grinding wheel; and after the roundness and straightness meet the technical requirements, the surface roughness of the slip ring can be achieved by fine grinding via a fine grinding wheel until the surface roughness meets the technical requirement. The field processing method has the characteristic of short maintenance period, is simple and feasible to operate, and is safe and reliable in performance.