Abstract: The echolocation method for measuring the nitrogen-brine interface during salt-cavern solution mining is advantageous from the aspects of easy operation and low cost, and it can control the cavern shape effectively. However, its measurement accuracy is not high for the annulus condition in the process of salt-cavern solution mining is complex and the waveform recognition is difficult. To recognize the waveform of nitrogen-brine interface accurately, a numerical model to measure the nitrogen-brine interface during salt-cavern solution mining based on the echolocation method was established by the software Gambit. Then, the waveform of nitrogen-brine interface measured by the echolocation method was simulated numerically by the software Fluent. And finally, the waveform characteristics of the nitrogen-brine interface were obtained for three annulus sizes, i.e., fixed, reduced and enlarged. It is shown that the pressure wave curves have distinct characteristics in different annulus states, so based on the characteristics, the nitrogen-brine interface during salt-cavern solution mining can be measured accurately by means of echolocation method. The research results can be used as the theoretical and practical guidance for the interface measurement of salt-cavern solution mining by the echolocation method.