Abstract: Proposes an agent intention theory based on nonstandard modal logic. When resolving side effect problem, the theory introduces the concept of scenario to interpret the semantic meaning of intention operator and uses new inference rules to replace the old ones. In order to describe relations between intentions, the theory first categorizes intentions into three classes: primary intention, relative intention and atom intention. Then it constructs the three classes of intentions into a tree. Relation operations are introduced together with their semantic meaning. Along with the intention tree, the paper also proposes the concept of intention power as an innate characteristic of the agent intention. Use of intention power can help the agent to improve greatly its dynamic efficiency.