Abstract: The distorted shape of array is one of the main reason, which results the performance degeneration from the ideal situation of towed line array. Based on the ordinary array shape distortion, the directivity function of towed line array with distortion is presented in this paper. An algorithm for precisely determining the coordinates of each element by means of the inverse elliptic function is derived. A fast approximation of recursive formula for solving distorted array shape is given. According to the comparison of ideal directivity and directivity of distorted array, a criterion for making decision of operational mode of towed line array is presented. So that the performance prediction problem in towed line array is solved. The results of system simulation in computer shows a good agreement with the theoretical analysis.