Abstract: This paper mainly studies on the refining category of belief, and the cognitive properties about intelligent agent with rational belief. Based on the semantic analysis of the rational belief, we establish a cognitive inference system. After the proof of its soundness and completeness, we find rational belief is a necessary condition for intelligent agent to get knowledge, and also a limiting condition for knowledge definition. This condition is also a limiting condition for the definition of knowledge which was neglected in previous. What is more, it is one reason for the different between intelligent agent and biological cognitive agent in cognitive inference.