Thermocline is an important ocean phenomenon. In this paper, we present a thermocline detecting and tracking method based on adjustable thermocline grad threshold by AUV (autonomous underwater vehicle), the threshold can be self-adaptively modified according to thermocline distribution characteristics, meanwhile, for the key issue of bin size setting in thermocline tracking, an autonomous algorithm based on the number of grad peak points is proposed, which can make AUV completely self-determination. With data of AUV filed test, comparative analysis is proceeded with Peak-Gradient, Average-Gradient and Optimal-Segmentation methods, the result shows that the method is simple and effective.