Abstract: Effects of the distortion upon left/right discrimination of towed twin-line array were analyzed.Two key parameters,the characteristic bearing and characteristic frequency,were derived and defined according to the directivity and the gain of left/right suppression of twin-line array,which determine the performance of left/right discrimination of towed twin-line array.A shape distortion variable was called to formulate the left/right suppression ratio under shape distortion,and the functional relationship between the two key parameters and the shape distortion variable was established.Theoretical analysis and simulation results reveal that the characteristic bearing and characteristic frequency are deviated from original location under shape distortion,resulting in the performance degradation of the sonar left/right discrimination,and even worse bearing ambiguity would appear again.And the sea trial data verified this.