The steel - concrete composite structure has many advantages in the field of reinforcement of bridge. But it is an important factor that whether the stud connector can make the components be firmly combined and make them work together. The research in this area has been not enough depth. The purpose of this study is to establish the design formula of stud in composite structure. Based on the conversion section theory, the strengthened part of composite structure is simplified into a kind of structure which is composed by only one kind of equivalent material and its physical property is similar to the original. In addition, the simplified model is gotten by mechanical analysis of the composite structures with steel plate and stud. Then through the application of elasticity theory method, the static equilibrium is carried out on the integrated structure and so the shear capacity in stud of connectors is gotten.