The invention discloses a drop hammer impact testing machine, which comprises an iron block, a drop hanger, a wire cable, a grapnel device and a rail holding device. The invention is characterized in that a beam, an upright post, a cross arm and a support are fixed to form a frame of the impact testing machine via screw bolts; a pulley is mounted on the beam and below the upright post; one end of the wire cable is arranged on the beam, and the other end thereof is connected with a power unit through the drop hanger and the pulley; the drop hanger is provided with a drop hanger device for catching and releasing an impact hammer; the rail holding device is fixed in the middle position below the upright post. By using the invention, automatically catching the hammer and automatically decoupling can be realized while the rail cannot change position to finish repeated impact tests, thus saving testing times, reducing labor strength and improving work efficiency greatly.