Abstract: The motion law of a USV with 4 azimuth thrusters and the forces on the USV in 3 dimensions (surge, swaying and yaw) are analyzed and the model of the motion is built.The linear response equation of the USV is derived.Fore typical operation modes of the thrusters are simulated:1 only two bow thrusters operating, 2 only two stern thrusters operating, 3 four thrusters all operating, 4 only two starboard thrusters operating.The tuning performance is focused on in the investigation.The simulations indicate that the best turning performance is achieved in operation mode number 4, with turning radius of 8 m.In contrast, the turning radius for operation mode number 1 is as worse as 50 m.