The invention discloses a tool for testing the performance of an ultrasonic flaw detector, and a testing method thereof. The tool comprises a pressing block, an elastic limiting piece, a plurality of adjusting pieces and a fixed frame; a through hole matched with a testing block is formed in the middle of the fixed frame; a sunken accommodating cavity is formed in the upper side of the fixed frame; the plurality of adjusting pieces are arranged on the side of the fixed frame at equal intervals; a blind hole matched with an ultrasonic probe is formed in the lower side of the pressing block; a through hole which runs through the pressing block is formed in the upper end of the blind hole, and the hole diameter of the through hole is matched a wire of the ultrasonic probe; the elastic limiting piece is shaped like a ring made of an elastic material; and the elastic limiting piece sleeves the upper end of an ultrasonic probe base, is positioned in the through hole of the pressing block and is used for filling a gap between the pressing block and the ultrasonic probe. The problem of the influence on the testing result by the facts that the highest echo is searched difficultly by the probe, the human factor influence in the testing process is big and the coupling consistency is difficult to guarantee in the existing testing method is solved.