Numerically simulating a ship with six-degrees-of-freedom response motions of an unsteady maneuver in a wave environment is very important in seakeeping characteristics of ship design. This paper presents the simulation studies of the turning motion in regular waves of the ONRT model. Numerical simulations were performed using viscous CFD code HUST-Ship to solve the RANS equation coupled with six degrees of freedom (6DOF) solid body motion equations and dynamic overset grids designed for ship hydrodynamics. RANS equations are solved by the finite difference method (FDM) and PISO arithmetic. The level-set method is used to simulate the free surface flow. Before the turning circle simulation, a V&V study is conducted for the total towed resistance. The real propeller was replaced by a description body force method in the process of turning motion. The constant rate of the revolution was applied throughout the simulation. The rotation of the propeller corresponds to the self-propulsion point of the model speed. The control of rudders was controlled by the following autopilot. The maximum rudder rate was assigned to 35.0 [deg/s]. The ship was released when a wave crest is passing the midship. The study focused on the parameters of the trajectories for turning circle, roll, pitch, velocity, etc, it is helpful to judge the influence of the wave on the turning motion. The simulation results match well with test data from IIHR.