Unpublished dissertations belong to gray information sources. Dissertation is a research report or scientific paper written by the author to obtain a certain degree. It is generally divided into three levels: bachelor's thesis, master's thesis and doctoral thesis. Among them, the doctoral thesis is of the highest quality, which is an original scientific research work and the focus of collection and utilization. Dissertation represents different levels of knowledge and is one of the important sources of literature information. Generally, it is not published in publications, and can only be obtained through degree-granting units, designated collection units and private channels.