Abstract: In order to solve the complex mechanical problem of thin-walled box girder of Y-shape bridge, the authors proposed a spatial finite element with 10 degrees of freedom for every node of the thin-walled box girder by increasing the degrees of freedom based on the theory of elementary beam. Meanwhile, the authors selected a typical Y-shape bridge from existing Y-shape bridges as a model example, the organic glass model of the typical Y-shape bridge was built according to the similarity theory. Monitoring points were placed on the experiment model with different supports and under different loading cases. In addition, the authors built the model of a spatial shell and the model of an elementary beam, and compared the results of the theoretical analysis with the experimental results. The analytical results show that the theory of elementary beam can not reflect the mechanical behaviors of the structure exactly, and the theory of shell has substantial weakness in the analysis of the whole structure, the result also indicates that this paper’s method is correct and simple. The research results provide a new method to solve the problem.