The high-pressure anechoic water tank is an important experimental testing platform for the development of deep-sea transducers, sensors, and other acoustic instruments and equipment. In this paper, background noise and acoustic field fluctuations at different frequencies were measured for the homemade 20 MPa high-pressure anechoic tank. The echo interference level under fixed measurement position and distance conditions was calculated, and the echo interference curve was drawn. The time-frequency characteristics of signals under typical low-frequency and high-frequency conditions were analysed. The measurement results of background noise show that although the background noise inside the tank is relatively high and has characteristic peaks in the frequency range of 10–12 kHz, it allows for measurement experiments with sufficient signal-to-noise ratio conditions. Meanwhile, the time-domain waveform results of sound field fluctuations measured in different frequencies show that the signal amplitude rapidly decays after a transmission width of 2 ms, and the higher the frequency, the faster the attenuation, indicating that the sound absorption cone inside the tank has a good sound absorption effect. The calculation results of echo interference level show that most frequency points above 10 kHz do not exceed ±1 dB. The designed fixed measurement position meets the requirements of free field testing, especially the echo interference of frequency points such as 20 kHz, 28 kHz, and 34 kHz does not exceed ±0.5 dB, which meets the requirements of precision measurement.