The main purpose of this paper is to solve the yawing problem of a single point mooring catamaran which designed for some special offshore operations. Dynamic characteristics of mooring system had been analyzed with the effect of wind, wave and current in early research. The results show that the yawing motion would be occurred for that system and can not be avoided without external control methods. These results are supported by sea tests of the real catamaran. Then, an azimuth thruster, set aft the ship, is used for a simple dynamic positioning (DP) system of the catamaran. The system is not a complete DP system because only one freedom can be controlled for the horizontal plane motions of the catamaran. The yawing angle is selected as control variable to meet the design requirement. The thrust is perpendicular to the longitudinal axis of the ship and can provide a maximizing yaw motion for DP system. Proportional-derivative control law is applied and the feedback variables can be obtained by global positioning system in further practices. The control effectiveness is validated by numerical results and comparisons with the non-controlled situations.