In order to study the relationship between chemical structure and antipetic ulcer activities of pyridine aldehyde N⁴-substituted phenyl(thio)semicabazones,the ab initio RHF calculation was undertaken and their electronic structure was obtained.Conformation analysis was carried out to study their structure.Their active site and the way of action were determined.Their structure-activity relationship and the interaction with receptors were discussed.The results showed that the activities of these compounds were strongly dependent on energy and their composition the Highest Occupied Molecular Orbital.From these,we can conclude that the pyridine and carbonyl group were active site in these compounds.And when these compounds reacted with the receptors,they would offer electronics.These wound give clues for further molecular design.