Unmanned and intelligent target detection and tracking is the inevitable trend of underwater exploration in the future. An automatic tracking method for small moving underwater targets is proposed based on angle and distance information received by fixed sonar. Firstly，the image sonar is processed with constant false alarm，the time-domain parameters are introduced，and the weight of previous time is added to the threshold calculation to suppress the static strong background interference with constant position. And then，based on the continuous motion of the target in space，the automatic tracking algorithm of small target is proposed. Experimental processing of lake test data shows that this method can effectively suppress interference and realize automatic detection and tracking of small underwater targets.
