[en] In this paper, the modeling and simulation research of the third loop system of CEFR using the GSE company's graphic modeling tools Jtopmeret is introduced firstly. According to the characteristics of the small water volume and the wake capability of saving heat, uncertainty and non-liner of the mathematics model, and hard to control of the exit steam pressure when load changed or disturbed, the immune control method is introduced. Based on the Sigmoid non-linear model by gradient search method and hill function. The simulation results in once-through steam generator pressure control show that the controller has small overshoot and shorter steady time. (authors)