The utility model discloses a machining clamp for a main water return pipe. Fixture base plate, thin air cylinders are arranged at the two ends of the main water return pipe and the outer side of themain water return pipe on the clamp bottom plate. A rotary pressing plate is arranged on each thin cylinder; a plane positioning column is arranged at the top end of the rotary pressing plate, three lateral positioning supporting bases are arranged on the two sides of the main water return pipe, five threaded locking type elastic supporting bases are arranged below one side of the main water return pipe, an adjustable supporting piece is arranged on the other side of the main water return pipe, and an auxiliary pressing plate is arranged on the adjustable supporting piece. The lever-type rotary air cylinder is used for compressing a workpiece, pressure is even, and machining is stable. The thread locking type elastic support is adopted, the stability of the auxiliary support is improved, the workpiece clamping time is short, and the production efficiency is high.