The invention discloses a locating mechanism for 45-DEG machining of a work piece. The locating mechanism comprises a rectangular chassis, an L-shaped limiting member, a rotating shaft, a handle and a U-shaped work piece mounting mechanism, wherein the work piece is fixed on the U-shaped work piece mounting mechanism through a fastener; the rotating shaft is arranged in the centre of the bottom of the U-shaped work piece mounting mechanism; the bottom of the rotating shaft is plugged on the rectangular chassis; the rotating shaft is provided with a connecting rod; one end of the connecting rod is provided with the handle; the rectangular chassis is provided with the L-shaped limiting member; the two sides of the L-shaped limiting member form an angle of 90 DEG; the side edge of the L-shaped limiting member and the side edge of the rectangular chassis form an angle of 45 DEG; the U-shaped work piece mounting mechanism is leaned against behind the L-shaped limiting member; and the side edge of the U-shaped work piece mounting mechanism and the side edge of the rectangular chassis form an angle of 45 DEG. Through simple operation of the locating mechanism for 45-DEG machining of the work piece disclosed by the invention, the work piece can be machined for an angle of 45 DEG from left to right, so that the efficiency is extremely high.