The invention relates to the technical field of machining technical equipment, in particular to a two-face finish turning clamp for a large thin-wall flange and a flange machining process. The two-face finish turning clamp for the large thin-wall flange comprises an upper permanent magnet disc, a lower permanent magnet disc, a non-magnetic middle connecting disc and a plurality of rotary positioning claws made of magnetic materials. The upper permanent magnet disc and the lower permanent magnet disc are fixed together through the middle connecting disc, and an annular groove space is formed among the lower end surface of the upper permanent magnet disc, the upper end surface of the lower permanent magnet disc and the outer circle of the middle connecting disc. Each rotary positioning clawis provided with a rotary shaft, an upper positioning claw body and a lower positioning claw body and is installed in the position, in the annular groove space, between the upper permanent magnet discand the lower permanent magnet disc through the rotary shaft, wherein the upper positioning claw body and the lower positioning claw body are arranged in opposite directions around the center of therotary shaft. By means of the two-face finish turning clamp for the large thin-wall flange and the flange machining process, rapid switching of flange lower end surface positioning and flange upper end surface positioning can be realized, the flange does not need to be hoisted or turned over, and the flange is high in machining efficiency and good in machining accuracy.