Aiming at the precise positioning of autonomous underwater vehicle(AUV) during submerged operation for long periods of time,an underwater terrain matching positioning method based on maximal likelihood estimation(MLE) is proposed.In this method,multibeam echo sounder(MBES) is taken as the underwater terrain measuring means,and maximum likelihood algorithm is used for correlation analysis of terrain character.For the influence from false peaks of likelihood function at flat bottom area,the Fisher criterion is introduced to restrict it.The false peaks are eliminated effectively and the discrimination of flat bottom area is enhanced.The simulation results based on one electronic chart data and sea test data from a multibeam echo sounder show that the proposed method is effective and feasible.