Five mono cycle β Lactams’ structures were optimized by semi empirical AM1 method of quantum chemistry.The ring opening reaction mechanism of mono cycle β Lactams in the presence of base were studied.The geometric configurations of reaction transition states and reaction products were calculated.At the same time,the law of electronic transference in reaction was found. Five mono cycle β Lactams’ structures were optimized by semi empirical AM1 method of quantum chemistry. The ring opening reaction mechanism of mono cycle β Lactams in the presence of base were studied. The geometric configurations of reaction transition states and reaction products were calculated. At the same time, the law of electronic transference in reaction was found.