The purpose of studying combinatory mathematics is not only to calculate the answer, but to understand the process of calculating the answer. In this paper, this study attempts to use the combined method to prove the following equation: C(r,n)*(n-r)*C(r,(n+r-1))=n*C(2r,(n+r-1))*C(r,2r) To solve this combination equation, instead of using the general expansion calculation method, two sets are constructed whose numbers of elements are, respectively,C(r,n)*(n-r)*C(r,(n+r-1)) and n*C(2r,(n+r-1))*C(r,2r), then a function are constructed between two sets. This function is characterized by a one to one and onto, that is to say this function is a bijective function. We can use this method to complete the proof of this article.