The present invention discloses a ultrathin acoustic impedance converter belonging to the acoustic field, which is characterized by comprising one or a plurality of impedance conversion units, wherein each impedance conversion unit is composed of a frame, a plurality of prestressed membranes or prestressed string nets, and multiple layers of acoustic materials, wherein a through cavity is fabricated in the frame, the prestressed membranes or string nets and the acoustic materials are alternately arranged in the cavity, i.e., a prestressed membrane or string net is arranged, and then a layer of acoustic materials is arranged, and so on until the through cavity is fully filled. The cavity can be designed in different shapes either with a variable cross section or a uniform cross section. Each prestressed membrane or string net is required to be applied with prestress before being arranged in the cavity, and the magnitude of the prestress depends on the acoustic impedance value that the membrane or string net is expected to reach. The novel ultrathin acoustic impedance converter of the present invention can realize rapid change from low impedance to high impedance or from high impedance to low impedance and realize ultrathin design.