This study aims to measure the space charge of extremely thick insulating systems, such as full-scale ultrahigh-voltage cables and joint boxes, with high accuracy using the pulse electroacoustic method. The proposed method employs an electrically insulating acoustic coupler to improve acoustic matching. Additionally, a metallic backing with high acoustic impedance is applied to upgrade the sensitivity to almost five times that of conventional measuring systems. However, the proposed structure requires an extremely thick backing material to avoid multiple reflections when the specimen thickens. In this study, a backing structure that combines a thin polymer plate with an air layer is proposed. Because multiple reflections are not returned from the air layer, the backing structure can be designed to be extremely small, thereby maintaining high sensitivity. This advantage is verified experimentally.