Abstract: An optimal algorithm for single-mode close-loop excitation in shallow water is presented. By analyzing the covariance of estimation value of Green's function matrix, an optimal source array weight matrix is presented to estimate Green's function matrix. The weight matrix is a unitary matrix, and absolute values of the matrix elements are equal. Algorithm based on the weight matrix makes single-mode excitation converge at maximum speed and be steady. Advantages of the algorithm are confirmed by numerical simulations. Finally, results of shallow water experiment are presented, and the energy ratio of single mode is higher than 97 percent.