This work presents results of direct numerical simulations (DNS) of a one-bladed vertical-axis wind turbine (VAWT). The flow field around the turbine blade is simulated at two different tip-speed ratios λ = 1.5 and 3.0, to capture different dynamic stall scenarios. The phase-averaged flow fields and unsteady aerodynamic loads obtained from the DNS are compared with the experimental measurements. The results demonstrate a high level of consistency in the flow field development and the evolution of aerodynamic coefficients. However, the high-fidelity simulation additionally captures the interaction between the dynamic stall vortex and the turbine blade, which was not seen in experiments due to the three-dimensional effect from the tip vortices of turbine blade and the background disturbance. The interaction of separated dynamic stall vortex and turbine blade, as well as some smaller vortices generated at the upwind side, convect downstream and interact with the blade at the downwind side. This contributes to the difference to the experimental results, such as the total force coefficient having the second peak in the downwind cycle. The proper orthogonal decomposition (POD) analysis not only provides detailed insights into the three-dimensional flow structures around the turbine blade but also facilitates comparison with the aerodynamic loads, offering a clear indication of the dynamic stall evolution on the vertical-axis wind turbine. The high-fidelity simulations and modal analysis of the VAWT provide deeper insights into dynamic stall phenomena and help identify potential strategies for improving performance through enhanced flow control methods.