This paper presents a design method of dynamic positioning control system for floating platform with rotatable and retractable thruster using $H_{infty}$ control technique. The norm band of uncertainty is captured by multiplicative perturbation between nominal model and reduced order model. A controller robust to the uncertainty is designed applying $H_{infty}$ synthesis. The control law satisfying robust stabillity and nominal performance condition is determined through the mixed sensitivity approach. The evaluation for the resultant controller obtained by $H_{infty}$ synthesis is done through simulations of the closed loop system. The results of $H_{infty}$ synthesis are compared to those of the traditional LQ synthesis method.