In this paper,we use addition theory on the Abelian group to study two problems on the Abelian group, the direct problem ,that is, let A and B, what are properties and the structure of the sumset A+B? , and the inverse problem- when |A+B| is as small as possible, what are properties and the structure of the set A and the set B?The main work of this paper has three aspects. Firstly, we introduce a new result of the direct and inverse problems on the Abelian group; secendly, we deduce computation on |2A| in some special cases; lastly, we useaddition theory on the Abelian group (za0,+) to improve Vitek's bound andShen Jian's bound in two special cases, and obtain some new estimated formulas for the Frobenius numbers.This paper is organized as follows:Chapter 1 Introduction. We introduce the main problem of study on additive number theory and the main result of this paper.Chapter 2 The sumset on integers. We introduce mainly a new result on the direct and inverse problems of the sumset on integers, and we deduce computation on |2A| in four special cases.Chapter 3 Some new estimated formulas for the Frobenius numbers. We use addition theory on the Abelian group (za0,+) to improve Vitek'sbound and Shen Jian's bound in two special cases, and obtain some new estimated formulas for the Frobenius numbers. |