Abstract: The impact of the multi-layer, unequal-spacing oil gap structure in the oil-pressboard composite insulation of converter transformers on the oil electric field and interface charge remains unclear. This study employs the Kerr electro- optic effect to acquire the electric field distribution in oil and the interface charge accumulation characteristics in a typical two-layer oil- pressboard composite insulation model under different applied voltages. It reveals the "oil gap spacing effect" under the combined influence of "high oil field strength" and "wide oil gap spacing". The study shows the following findings: 1) When the electric field strength in the oil is low, the electric field strength on the positive polarity side of the oil is consistently higher; 2) As the applied voltage increases to +25 kV, and the ratio of the upper/lower oil gap spacing increases from 1:5 to 4.5:1.5, the electric field strength on the positive polarity side of the oil gradually decreases, with a field strength distribution "reversal" occurring once this ratio exceeds a certain threshold; 3) The threshold for the "reversal" of the electric field strength distribution in adjacent oil gaps is clarified: when the spacing ratio is 3:1, the threshold electric field strength in the oil is 1.00 kV/mm, and when the spacing ratio is beyond 5:1, the threshold electric field strength reduces to 0.55 kV/mm; 4) The wider oil gaps provide more initial charge in the oil, and the intensified electric field exacerbates charge dissociation and recombination, resulting in an increase in the interface charge on the oil-pressboard interface on the wide oil gap side, surpassing the "polarization effect, " and thereby enhancing the electric field strength in the opposite oil gap. The research findings provide theoretical and technical support for the optimization of the design of oil-pressboard insulation structures in converter transformers.