In this paper, the concept of action oriented belief updating is introduced based on the PMA (possible model approach). It is proved that a first order formula holds in a situation if and only if it belongs to the belief set of the situation provided the situation calculus system is informational complete. This result makes it possible to avoid the appeal to second induction axiom in the entailment of situation calculus and also provides a theoretical foundation for modeling agents with intention driver and the agent-oriented programming language AOPLID (agent-oriented programming language with intention driver).