This paper presents a design method of dynamic positioning control system(DPS) for floating Platform with rotatable and retractable thrusters using H$infty$ servo control design method. 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 stability and nominal performance condition is determined through the mixed sensitivity approach. The control algorithm was evaluated on the basis of computer simulation for a proposed DPS design method and experiments was carried out with an image processing method for measurement of DPS position in a water tank The results of overall experiments show that proposed control method will be good to keep at a specified position. And they are compared with the experimental results by LQG synthesis and H$infty$ optimal control design method.