The fatigue damage on wind turbine blades will threaten the safety and stability of the wind turbine and thus lower the efficiency and economy of the power generation system. The wake-induced fatigue plays an important role in this fatigue damage, which has not been deeply studied in the previous research of this domain. This is because the calculation of fatigue damage on a wind turbine blade under wake conditions will include the knowledge of wind turbine wake (fluid mechanics), composite structure modelling (solid structure modelling), aero-elastic modelling (coupling between fluid and structures), and fatigue analysis. Moreover, the anisotropic composite wind turbine blade, e.g. bendtwist coupling blade, will also bring challenges for the structural modelling. To propose a model to solve the above problems holistically is the motivation of the thesis. In this thesis, to construct the aero-elastic model under wake conditions for fatigue analysis, the elastic actuator line model is proposed and verified. To consider the anisotropic properties of composite wind turbine blade, e.g. bend-twist coupling wind turbine blade, the anisotropic wind turbine blade structure model is constructed. Based on the structure model and cross sectional analysis method (BECAS), the fatigue analysis methodology is proposed. Due to the similarity between the anisotropic wind turbine blade structure model and Maxwell's equation (electromagnetic equations), the FDTD method, which is a FDM based method and long been used in electromagnetic simulation, is applied to construct a novel anisotropic wind turbine blade structure model. Specifically, firstly, the actuator line model is validated in terms of thrust coefficient and flow field prediction. It is found that the nacelle effect has impact on the velocity profile around wake center region. And the proposed single-point nacelle model, single momentum source point smeared by Gaussian function, can be used to correct the prediction not only for RANS turbulence model but also for LES turbulence model. Secondly, Based on NREL SOWFA, the elastic actuator line model is constructed as an aero-elastic model for wake conditions to simulate the dynamic loading of wind turbine blade. The stochastic and deterministic wake-induced fatigue loading are reproduced by the proposed elastic actuator line model. Compared with the explicit elastic actuator line model, the implicit elastic actuator line can run with larger time step. However, the accuracy of implicit method will decrease. Thirdly, the performance of normal and bend-twist coupling wind turbine blade with anisotropic composite materials in wake conditions are studied by using the fatigue analysis methodology based on anisotropic structure model, cross sectinal analysis, and fatigue analysis method. Based on this fatigue analysis methodology, the fatigue life of NREL 5MW wind turbine blade is analysed. The predicted fatigue life (26.0187 years) of the main structure (spar caps) is very close to the design life (20 years). From the fatigue analysis for wind turbines in wake conditions, it is found that the wake-induced fatigue has a significant impact on the fatigue life of wind turbine blades (fatigue life drops from 26.0187 years to 1.7388 years under compact layout). And wind farm layout can affect the wake-induced fatigue damage (increase from 1.7388 years (compact layout) to 6.9084 years (normal layout)). Furthermore, it is also found that the bend-twist coupling wind turbine blade can alleviate the fatigue load under wake condition. Lastly, the structure model based on FDTD method is constructed for anisotropic wind turbine blade and validated in terms of beams with deformation coupling, non-inertia coordinate system, and non-uniform sections (real wind turbine blade). The stability analysis for the proposed FDTD model is carried out, which shows that the root cause of the numerical instability for the proposed method is the highest-frequency mode in numerical model. Based on this analysis, the unconditionally stable explicit FDTD structure model is proposed and constructed, which strikes a balance between accuracy and efficiency. Compared with implicit method, the unconditionally stable explicit FDTD model overcomes its limitations on time step with little effect on its solution accuracy.