Uplift of the horizontal rectangular anchor plate is a typical three-dimensional problem. However, it is difficult to characterize the sliding surface of soil around the anchor plate in the limit state for its shape is affected by a combination of the length-width ratio and embedment ratio of the anchor plate. Combining the results of ABAQUS 3D numerical simulation and 2D sliding surface analysis, it is found that the geometric form of the 3D sliding surface in any horizontal section within its buried depth range can be described by a closed graph, including four segments of straight lines parallel and equal to the long and short sides of the anchor plate respectively and four segments of 1/4 arc. The horizontal distance between the straight line and the corresponding anchor edge is determined by the 2D sliding surface in the vertical symmetric plane in the center of the long edge. The shape of this 2D sliding surface depends only on the embedment ratio, and it can be characterized by the logarithmic spiral morphological function. Based on these understandings, a three-dimensional mechanical analysis model of uplift bearing capacity of the horizontal rectangular anchor plate was constructed for the first time, and four cases of the model were analyzed. Combining the decomposition and merging of the model, the mechanical limit equilibrium analysis of the isolation body was carried out. Then, the calculation method of uplift bearing capacity of the horizontal rectangular anchor plate was deduced. It is applicable in the whole range of the length-width ratio and embedment ratio. Compared with five test cases and three other calculation methods, the results show that the proposed method has the best performance in all kinds of sand ground with various relative densities, which shows good applicability.