The invention provides a desired path construction method in under-actuated AUV underwater dynamic target tracking. The method includes the steps that firstly, an estimated value of a target positionat a current moment is obtained through an interactive multi-model method, and the observation value of the target position at the current moment is obtained through an ultra-short baseline and binocular vision positioning system; secondly, the optimal estimated value of the target position at the current moment is obtained through Kalman filtering; thirdly, the position of an AUV, a selected historical target point sequence and the optimal estimated value of a target at the current moment are selected to serve as an expected target point sequence; geometrical constraint conditions are introduced, a cubic spline curve is constructed according to the expected target point sequence, and the spline curve is selected to serve as a desired path curve of AUV tracking. According to the method, the desired path curve can be updated in real time, the influence of uncertainty of the AUV for handling the target and instantaneous interference is improved, and the tracking rapidity and precision are improved.