Abstract: The ground level ion current density produced by corona discharge in high voltage direct current (HVDC) transmission lines can reflect the operation status of the lines, but the distorted electric field at the edge of the Wilson plate seriously affects the measurement results of ion current density. In order to measure the ground level ion current density accurately and directly, a new reduced scale wire-plate experimental device in which the Wilson plate is flush with the grounding plate is designed. The influence of protective annulus width and the height of the Wilson plate from the grounding plate on ground level ion current density are studied. In addition, the differences between the micro-current galvanometer method and the sampling resistance method in the measurement of ion current density are compared. Finally, the ground level ion current density distributions of unipolar and bipolar HVDC transmission lines are measured. The results show that the edge effect of the Wilson plate can be neglected when the width of protective annulus is less than 2 mm, and the ion current density is nonlinear with the height of the Wilson plate from the grounding plate. Moreover, the internal resistance of the digital voltmeter seriously affects the measurement results and it is necessary to correct the results. Finally, at the same applied voltage, the ground level ion current density in the negative conductor region is higher than that in the positive conductor region.