Abstract: In this paper, a technique for calibrating a towed-line array or one of its groups of elements in a long tube is described. In this technique, the complex senstivity and far--field directivity of line -array are measured by producing a field that is equivalent to a free-field plane wave of any incidence on the array. First, the fundamental principle of this technique is presented. Then the applicability of the technique and the relation between the quality of the simulated field and the parameters are investigated by computer simulation after a theoretical model is set up. Finally, the conclusion are tested by experiments. The measured sensitivity and pattern of a four-elements line array and the measured pattern of a two-elements array have shown that this technique is a very efficient method for calibrating long-line array in low frequency.