Aristotelian syllogism is traditional logic which is considered as un-formalized normally. The paper formalized Aristotelian syllogism as the forms of propositions with all the rules for getting the valid forms (figures).Based on the results, an automatic system are developed using VC++ and MFC(Microsoft Foundation Class). For any Aristotelian syllogism which are divided into 4 types named as 4 figures and numbered totally 256, the valid conclusions can be output when users input the premises of a Aristotelian syllogism. The core codes and the methods of transforming Aristotelian syllogism logic into programming logic are presented.