Defects at the leading edge of blades are an important source of power loss for wind turbines. In the present work, a systematic study is carried out on two typical types of defects, i.e., the surface concaved deep defects and surface distributed shallow defects on the 5809 airfoil. Different defect shapes, ranges, equivalent depths and their influence mechanisms are investigated via CFD. For deep defects, an enclosed vortex is formed in the defect cavity, which suppresses the momentum exchange between the external flow and internal flow, so that the airfoil aerodynamic performance is highly sensitive to the defect opening range and is little affected by the defect shape and equivalent depth. Flow around the leading edge is strongly hindered by the deep defects and an elongated leading-edge separation bubble is formed at the suction side of the airfoil. At large angles of attack, flow at the suction side is dominated by flow separation at both the leading edge and trailing edge. In contrast, for shallow defects, all the defect equivalent depth, opening range and shape can significantly influence the airfoil aerodynamic performance; and quantitatively, the effect of defect equivalent depth is the most significant. For the present simulation cases with defects, the maximum lift coefficient is notably decreased (by 35% to 61%) accompanied by a sharp increase in drag coefficient (by 131% to 217%). Under dynamic pitching motions, the opening of the dynamic lift (drag)-coefficient hysteresis curve is effectively enlarged. The present work aims to provide an important reference for the maintenance and management of turbine blade defects.