To address the insufficient accuracy of conventional isotropic analysis for large-diameter monopiles for offshore wind power and quantify the influence of soil anisotropy on monopile response, the depositional anisotropy was described by the K0 coefficient, and stress-induced anisotropy was modeled via the NGI-ADP constitutive model. Moreover, numerical analyses of monopile responses with varying length-to-diameter ratios were conducted. The soil layer was assumed to be 80 meters thick, and the pile head was subjected to a lateral load of 1 000 kN. Multi-condition calculations were performed to explore the impacts of K0 (ranging from 0.5 to 2.0) and lengthto-diameter ratio (ranging from 4 to 20) on monopile lateral displacement and shear stress distribution. Results indicate that when K0=1 (isotropic condition), the lateral displacement at the mudline is maximized. For K0≠1 (anisotropic soil), the lateral displacement is consistently smaller, and conventional isotropic analysis overestimates the lateral displacement by 30%~40%. When stress-induced anisotropy is considered, the maximum lateral displacement and displacement ratio of the monopile are significantly larger than those under the isotropic assumption. Moreover, this effect becomes more pronounced with increasing length-to-diameter ratio: displacement deviation rises from 42.9% (length-to-diameter ratio=4) to a peak of 48% （length-to-diameter ratio=10), then decreases slightly. The study concludes that in engineering design, it is essential to test the K0 value of the soil layer and incorporate anisotropic effects to improve computational accuracy and optimize pile diameter design.