Abstract: The researches on mechanical mechanism and bearing capacity of horizontal pullout of vertical square anchor plates have the problem of artificially distinguishing shallow and deep buried types without uniform definition standards. This study is devoted to the researches on the three-dimensional unified theoretical solution of horizontal ultimate pullout capacity of a vertical square anchor plate based on deep analysis of failure mechanism. The symmetry of failure mechanism varying with soil properties and buried ratios in vertical and horizontal directions is reflected by the evolution of projected triangles of rectangular pyramid soil core before the anchor plate to the vertical plane and horizontal plane, respectively. A three- dimensional unified mechanical model is established for the horizontal ultimate pullout of the vertical square anchor plate. The ultimate mechanical equilibrium analysis method is used for different loaded bodies in turns to derive the three-dimensional unified theoretical solution. Comparison with other theoretical methods and test data indicates that the new model can reflect the continuous variation rules of the symmetry of failure mechanism very well in different ranges of depth ratio. The three-dimensional unified theoretical solution has extensive applicability to the model test and field test data. The new solution performs the best as its calculation result is more close to the measured value with smaller discreteness and the average is generally safe.