Main steam pressure is an important physical quantity that reflects the energy supply-demand relationship between the boiler and turbine. It has a significant role in the unit operation. Because boiler burning behavior varies greatly and the model of main steam pressure is of highly uncertainty, conventional control method can not obtain the expected control effect. In order to improve system control quality, a robust controller for main steam pressure is designed by using the H infinity mixed sensitivity approach in this paper. To better meet the site requirements, adding more restrictions in design, author innovation to put such a complex issue into multi-objective optimization problems. SPEA2 (The Strength Pareto Evolutionary Algorithm 2) are used to optimize the parameters of weighing functions in order to search for the H infinity controller which meets the time-and frequency-domain indexes. Simulation results show that the design of the main steam pressure control system has an excellent robust stability and dynamic quality.