The invention relates to a positioning clamp for machining precision castings and a using method. The positioning clamp comprises the following steps: correspondingly inserting 12 projections on four projected positioning blocks of a positioning clamp body into 12 positioning grooves in a surface B of a case body of a case; attaching the bottom surface of the positioning clamp body to the surface B of the case body of the case; clamping the case on a horizontal milling machine and machining a surface A and a surface C of the case; then placing the case on a vertical milling machine and machining a surface D; before machining the surface B, successively placing a chiseling knife in a rectangular hand hole I, a rectangular hand hole II and two rectangular hand holes III of the positioning clamp body; beating a chiseling handle by using a hammer to reserve scratching marks on the left wall, the middle wall and the right wall of the case; then disassembling the positioning clamp body; respectively comparing the left wall, the middle wall and the right wall of the case with a left reference side wall, a middle reference side wall and a right reference side wall; and analyzing skewing of a machining reference of the surface B. A technologist can design a machining scheme by observing positions of the marks, errors after machining of the case are controlled in a reasonable range, and the machining efficiency is improved.