The composite bucket shallow foundation proposed by Tianjin University can be better adapted to the soft geological conditions in China for offshore wind engineering. The offshore wind turbines are generally subjected to relatively large horizontal load induced by waves, currents and ice loading. Therefore, calculating the horizontal bearing capacity is an important part of the design for the composite bucket shallow foundation. In this paper, according to the numerical simulation, considering the constraint of bucket foundation on internal soil in different degrees firstly, the horizontal soil damage rate is introduced into the formula as a new empirical parameter Next, upper-bound solution of the horizontal bearing capacity of a composite bucket shallow foundation is derived in sand. Fianally, the calculation method is validated by the bucket model tests with different height-diameter ratios in sand under horizontal loading.