The invention discloses a backing-independent type acoustic impedance test method for an acoustic coating. An acoustic coating sample is placed at the intermediate position of the tube body of a water-filled impedance tube, and a liquid column of a certain height is ensured on both the upper surface and the lower surface of the acoustic coating sample. One end of the tube cavity of the water-filled impedance tube is internally provided with a projecting transducer. Two hydrophones are arranged in the liquid columns on the upper surface and the lower surface of the acoustic coating sample, respectively. The four hydrophones are respectively connected with a data acquisition instrument. The data acquisition instrument is connected with a measurement control computer. The projecting transducer is driven to launch a stabilized broadband white-noise signal. The data acquisition instrument acquires the acoustic pressures of the hydrophones at the same time and stores the acquired acoustic pressures as a data file. The height of the upper liquid column is changed, and then the data of the acoustic pressures are measured and stored again. After that, the stored data of the acoustic pressures are read, and then the input acoustic impedance and the transmission acoustic impedance of the acoustic coating sample are calculated according to a corresponding equation. According to the technical scheme of the invention, the low-and medium-frequency test accuracy is ensured independent of a backing. Meanwhile, the backing can be re-stuck without taking out the sample. Therefore, the measurement process is simplified, and the measurement efficiency is improved.