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This paper presents stability analyses on a towed underwater vehicle (TUV) motion control system using movable wings. The control method utilized is an output‐feedback controller based on linear state‐feedback control and a high‐gain observer. In order to analyze the closed‐loop stability of the system, the singular perturbation method is employed. Not only the asymptotic stability of the system is proved, but also an estimate of its region of attraction is derived. To describe a region of attraction, we propose a method by employing a hyper rhombus, which is quite suitable for quadratic Lyapunov functions. Further, to obtain a less conservative estimate, a state‐space‐scaling method is devised and its efficacy is evaluated by comparing with the conventional estimation method. Then, results of regulation simulations with various initial conditions are provided, which explores the consistency and gap between the theoretical estimate and the simulation results, and the robustness of the control system. Finally, a depth‐tracking control system via switching controllers is constructed according to the results of the stability analysis which investigates configuration of multiple equilibria with different depths and their regions of attraction, and demonstration simulation results are presented to illustrate the validity of the perspective given by the stability analysis.