Abstract: Background equalization technique has been extensively studied in the field of frequency spectrum analysis.It has been proved an effective method of improving line spectrum extraction.In a multi-beam digital sonar system,the time/bearing display is the most important interface between signal processing module and console module.It hasbeen proved that the system gain is often lost in the interface between these two modules.Background equalizationtechnique can improve the overall performance of the time/bearing display window.The results of post-processingof multi-beam data is filtered each time before displayed on screen.It is shown that the algorithm of median valuefiltering combined with OTA (Order Truncate Average) method is an efficient technique to equalize the non-uniform,non-stationary background data.A variable length of window is adopted to match the different width of main lobe ofbeam patterns.The results of system simulation show that the algorithm described in this paper is efficient in the casethat the background of multi-beam data is not uniform.