The fixture comprises a base, a positioning seat and a fixing seat, the positioning seat is arranged at the center of the base, a plurality of groups of positioning grooves are formed in the edge of the base, the positioning grooves are circularly distributed along the surface of the base, and the fixing seat is arranged on the surface of the base. The fixing seat is arranged in the positioning groove, the fixing seat comprises a supporting seat, a supporting rod, a compression nut and a compression block, the supporting rod is arranged at the upper end of the supporting seat, the compression block is arranged on the supporting rod, and the compression nut is arranged at the upper end of the supporting rod and located above the compression block. The workpiece is centrally positioned through the positioning seat, and is radially clamped by the pressing block after the edge extends to the support, so that the problem of deviation easily caused by traditional single-point clamping is effectively avoided, and the clamping precision is remarkably improved.