Abstract: A method for the inversion of transmission losses for the bottom reflection loss is proposed on the basis of the theory of the smooth-averaged sound field. The unknown bottom reflection loss is parametrized in terms of linear interpolation. The procedure of the inversion bases on the criterion of the least square error in transmission losses between calculating and measuring. By using the Gauss-Newton iterative approach, the use of non-linear least square algorithm is equivalent to solving a sequence of linearized least square problems. The physical causes of instability of the inverse problem are discussed and the stability is improved by means of the Levenberg-Marquardt method. The applicability of the inverse method is demonstrated in some numerical and experimental examples.