Phase-pure polycrystalline Ba0.6K0.4+xFe2As2 with 0≤x≤0.1 were prepared using a one-step solid-state reaction method. We found that overdoping of potassium can improve the critical current density (Jc). High-field Jc for samples with x=0.1 is three times higher than that for samples with x=0. Overdoping of K has minimal effect on the critical transition temperature (Tc). Less than 0.5 K degradations in Tc was measured for samples with x=0.1. Transmission electron microscopy (TEM) revealed high concentration of dislocations in samples with x=0.1, resulting in enhanced flux pining. Further analyses on magnetization loops for powder samples confirm that K overdoping can promote intragrain Jc. Our results indicate that slight excess of K in Ba0.6K0.4Fe2As2 superconductor is beneficial to high-field applications.