The invention discloses a steel wire shear elongation test device. An air pressure switch is connected onto an air pressure pump through an air pressure hose; a pressure regulating valve is connectedwith the air pressure switch through the air pressure hose; a pressure gauge is arranged on the air pressure pump; one end of the air pressure pump is connected with a sample clamping device through the air pressure hose; anvil blocks are arranged on the sample clamping device. The test method comprises the following steps: setting a test pressure under a condition that no steel wire exists between the anvil blocks, activating the air pressure switch and adjusting the air pressure regulating valve, making table values, disconnecting the air pressure switch, and re-activating and checking the pressure; disconnecting the air pressure switch if the pressure is normal, continuously testing, loading the steel wire, applying a load less than 200N onto the steel wire, retaining a test pressure head around the steel wire to activate the air pressure switch, and applying a shear load until the steel wire is broken. Due to steel wire elongation values obtained when the steel wire is broken and when the steel wire is stressed by maximum pull, the shear elongation of the steel wire is calculated, and when the shear elongation is greater than 0.4%, the steel wire has excellent ductility.