The problem of synthesis of control system for once-through boiler is discussed in the paper. The simplified linear model of the boiler heater is considered here. It is proposed to use cascade control approach to control the plant as it is possible to measure the intermediate variable. Different dynamics of the inner and outer circuits simplifies the designing process. It is shown that the inner loop can be controlled by using typical P controller. Parameters of the external heater circuit are non-stationary and depend on the boiler load so it is suggested to use the robust PID controller to control it. The coefficients of this controller are suggested to bet calculated on the basis of the localization method. The main advantage of the method is using the derivative vector for manipulated variable calculations, which allows us to deal with the pending type of control object. This paper includes numerical experiments with a set of linearized model of the super-heater. The results show that the obtained control system has good enough quality of transient processes. The designed set of controllers was applied to the great non-linear model of the once-through boiler, which provides more realistic transient processes. The simulation results of this application are almost the same as in case of the linear model. Therefore it can be said that the designed control system fully perform all the requirements.