For the ship's maneuverability and seakeeping performance in waves, a numerical prediction method is proposed to study the maneuvering-seakeeping coupling motions based on dual time-scale, in which the ship-propeller-rudder hydrodynamics are estimated by empirical formula and the first order linear wave force and second-order wave drift force are calculated based on the three-dimensional potential flow theory and a database is established. Based on the presented numerical method, the turning motions of a ship in waves are studied and the time-step convergence of the dual time-scale method is discussed in detail. On this basis, the numerical test cases of three-factor and three-level typical conditions are established to study the essential reasons for the influence of the speed, wave frequency and wave height on the turning motions, and the turning motions are further divided into three typical modes according to the motion characteristics. The key characteristic parameters of ship maneuvering and seakeeping during turning motions in waves are discussed, and the optimal time-step principles of the dual time-scale method of various typical operating conditions are put forward.