The invention discloses a fixture belonging to the field of mechanical clamping devices. The fixture comprises two clamping arms, two clamping plates and a connector, wherein the two clamping plates are arranged between the clamping arms. The outer sides of the two clamping plates and the contact surfaces of the two clamping arms are arranged to be oblique surfaces, so that when the two clamping arms slide on the corresponding oblique surfaces, the contact area of the inner sides of the two clamping plates can be changed. In the process of the telescopic experiment of rubber sheets, the initial area of a rubber sheet clamped by the fixture is small, and the rubber sheet can be clamped to be thin enough by small clamping force; along with the gradual stretching of the rubber sheet, the two clamping arms slide on the corresponding oblique surfaces, so that the contact area of the clamping surface and the rubber sheet is increased gradually, namely, the area and the force to a workpiece clamped by the clamping plate are increased accordingly, so that the rubber sheet is clamped firmly.