In the design of monopile foundations for offshore wind turbines, in order to obtain a simplified calculation method for prediction of the accumulated rotation that can be used in engineering practice, the empirical formulae for calculating the accumulated rotation of monopile foundations under typhoon loading and long-term cyclic loading are obtained respectively by conducting laboratory model tests. On this basis, a simplified calculation method is further established. Taken an engineering case in the northern offshore area of the South China Sea as an example, the calculation process of the accumulated rotation of monopile foundations is described in detail. According to the characteristics of wind and waves in the sea area, the site-related parameters in the calculation formula are determined, and the formula for estimating the accumulated rotation of monopile foundations applied to the area is obtained.