The implementation of a moving surface in the aerodynamic profile of an airfoil is recognized as an effective active flow control mechanism. In this study, the effect of this mechanism on the aerodynamic and mechanical performance of an offshore horizontal-axis wind turbine blade at a Reynolds number of 7.5×105 was investigated. The S809 airfoil was selected as the blade section, and part of its suction surface was replaced with a moving surface. Simulations were performed using the commercial software ANSYS Fluent with the k-ω SST turbulence model, and the results were validated against available experimental data. To evaluate the influence of the moving surface length and speed, relative lengths (l) ranging from 0.05 to 0.55 chord and speed ratios of (k) 1,3 and 5 times the free-stream velocity were examined at a fixed installation location of s = 0.01 chord. The moving surface delayed stall angle by up to 80%, such that the drop in the lift curve shifted from an AoA of 8° to 12°, and the mechanical performance improved by a factor of five to seven depending on the AoA. At higher angles of attack, shorter lengths maintained flow control while reducing power consumption. In the aerodynamic analysis using the Blade Element Momentum method, the maximum power coefficient at a TSR of 4 increased by up to 4.6 times, while  the torque coefficient at the same point increased by 364%. The moving surface showed the greatest effect at low TSRs, improving both aerodynamic and mechanical performance.