The Backdoors as one of the important structures of problem QBF, it makes QBF problems easier in solving difficulty. In QBF problem, the solving of backdoors is quite complex and difficult to understand. In order to enable the reader to better understand the solving process, this paper researches the solving process of backdoors for QBF problems, calculates the depth of variable x and the triangle dependent set of variable D_ψ^Δ(x) that is meet the condition by combining the examples. This paper solves the backdoors of this problem according to the ideas of B=B∪{x}=x and ψ'=ψ-D_ψ^Δ(x). We are hoping that it can provide a reference for researchers in this field.