Intelligent system is a research field that attracts much attention at present. Most of the researches on intelligent system focus on intelligent technology and its application. However, an intelligent system is first of all a system, which means it should have the characteristics of a system. Design of conventional system is mainly function- or task-oriented, and adaptation to environment is passive, static and regular. However, intelligent system is faced with a complex, random and dynamic environment, and has dynamic interaction with the environment. Behind this interaction behavior is thinking and learning, and behind thinking learning is mission task and value. Therefore, intelligent system design needs to be oriented towards system behavior. This paper will examine intelligent system from the perspective of system behavior characteristics, and put forward a new definition of intelligent system. The core feature of intelligent system is intelligence. To construct an intelligent system, we have developed the concept of five core abilities of intelligence, which coincides with the traditional Chinese concept and way of understanding the world called "Unity of knowledge and action". Finally, we give a design case of an intelligent system (intelligent vehicle) to demonstrate the practical viability of the proposed architecture and concept.