Due to the nature of the atmospheric boundary layer, large Horizontal-Axis Wind Turbines (HAWTs) generally operate in highly unstable environment, leading to nonstationary loads on HAWT structures. Therefore, it is important to accurately estimate the nonstationary loads to ensure appropriate design boundaries. In this paper, an improved dynamic stall model based on the Beddoes-Leishman (B-L) model is proposed for the estimation of nonstationary aerodynamic loads. The B-L model is modified to account for the characteristics of wind turbine aerofoils which operate at lower Mach numbers and have a larger thickness-to-chord ratio compared to aviation aerofoils. Validation of the model is performed extensively through simulations of the S809 aerofoil under pitch oscillation with different mean angles of attack, oscillating amplitudes and reduced frequencies. To understand the effects of the modifications introduced to the dynamic stall model on the aerodynamic fatigue loads over the lifetime of a wind turbine, a load analysis of a 2 MW HAWT is conducted. The load analysis uses the Blade-Element Momentum (BEM) theory in conjunction with the dynamic stall models. The presented modified dynamic stall model is applicable to wind turbine aerofoils with relative thicknesses greater than 15%.