Operator support systems are extensively studied and developed to support human operators for their activities in especially an abnormal condition of a nuclear power plant. By the advancement of computer technology and artificial intelligence, an operator support system can provide detailed support information based on detailed models and utilizing detailed simulation of plant dynamics and/or complicated inference algorithms. However, human operators may not understand the detailed support information under high mental pressure in an abnormal plant condition. In such a case, it is important how to provide essential and understandable support information. This paper deals with a technique to simplify functional models in order to display operator support information that is generated based on detailed functional models. This paper defines eight cognitive states of human operators from the viewpoint of cognitive abilities of human. In addition, three ways to simplify functional models are identified.