Abstract: Currently, the research on bearing mechanism and capacity calculation of horizontal anchor plate has the problem of artificially distinguishing shallow and deep bury conditions without consistent standard. In this article, a logarithmic spiral shape function was proposed to describe the continuous evolution of the sliding surface of the soil around the anchor plate, and the unified mechanical model of ultimate bearing capacity was established with no need to consider the bury condition. According to the relationship among the buried depth of anchor plate, the distance from the sliding surface intersection to the anchor plate and the distance from the tangent point of vertical line of the sliding surface to the anchor plate, the limit equilibrium analysis method was used in three different situations to deduce the unified theoretical solution of vertical pullout ultimate bearing capacity of horizontal strip anchor plate. The results show that, the new mechanical model can well reflect the continuous variation of the sliding surface with the buried depth ratio, and no need to artificially distinguish the bury condition. In the comparative calculation and analysis of five test cases in loose and dense sand, the unified theoretical solution ranks the second among nine methods, and has good adaptability.