Converter transformer is one of the key devices in HVDC power transmission system. When polarity reversal takes place, space charges would not be able to quickly resolve but rather create its own electric field that superimposes on the newly established initial capacitive electric field after polarity reversal. As a result, a transient high electric field is triggered around the oil clearance,leading to partial discharge and even insulation breakdown that severely affect the safe and reliable operation of HVDC converter transformer. The present research used Kerr electro-optic method to have measured the electric field dynamics in oil at double polarity reversals and captured the interface charge accumulation characteristics for the oil-pressboard composite insulation.