The present invention relates to a jig for processing the inner surface of an aluminum alloy Winston cone baffle having the thickness of a sheet through an ultra-precision machining, the jig having a shape identically corresponding to the outer shape of a Winston cone baffle having a can body shape made up of compound parabolic, and divided into an upper plate jig and a lower plate jig in formation, wherein the upper plate jig is divided in two, a left side jig and a right side jig, which are formed to correspond in shape and size so as to enable isolation or coupling to/from each other, and the inner surface of the Winston cone baffle attached inside the upper plate jig is made to enable ultra-precision machining at the cutting speed of 220 m/min-300 m/min, which enables the inner surface of the Winston cone baffle to process a slickenside having approximately 4 nm of surface roughness, and in particular, ultra-precision machining at surface roughness of Ra=2.32 nm in a processing condition of cutting speed at 260 m/min, cutting depth at 4 micrometer, and feeding speed at 1 mm/min, thereby enabling formation of a Winston cone baffle through low-cost ultra-precision machining.