The invention discloses a test device of performance test of a stepped diffuser of a headbox, comprising a water storage tank, a centrifugal pump, an electromagnetic flowmeter, a steady flow box, a transparent stepped diffuser, a transparent water tank, a PIV test system and the like. The water storage tank is successively connected with a first ball valve, a first flexible single ball rubber elbow, the centrifugal pump, an adapter flange, the electromagnetic flowmeter, a second ball valve, a reducer union, the steady flow box, a first piezometer, the transparent stepped diffuser, a second piezometer and a second flexible single ball rubber elbow; the transparent stepped diffuser, the second piezometer, the second flexible single ball rubber elbow are arranged in the transparent water tank; the second flexible single ball rubber elbow is connected with the water storage tank; and the PIV test system is arranged on a corresponding part of the transparent stepped diffuser. Judgment and forecast are performed on rag rectifying performance of the stepped diffuser when flow velocity distribution and cavitation conditions at stepped parts of an axial cross section thereof are obtained.