Abstract: SF₆/N₂ mixed gas, which is used as a simple and effective environmental protection measure for SF₆, has been gradually applied to gas insulated power equipment such as gas insulated switchgear (GIS). A comprehensive study on discharge characteristics and synergetic effect under lightning impulse can provide a basis for insulation design and gas parameter selection, which has important academic research and engineering application values. In this paper, the discharge characteristics and synergistic effect of positive and negative polarity lightning impulse in quasi-uniform and highly inhomogenous electric field of SF₆/N₂ mixed gas were studied. It is found that, in a quasi-uniform electric field, there is a synergistic effect which is more significant under negative polarity. As the electric field non-uniformity increases, the type of synergistic effect changes from the synergistic to the negative synergistic. The method of synergistic effect coefficient C cannot fully describe the synergistic effect and negative synergistic effect of SF₆/N₂, therefore, an appropriate method of normalization coefficient h is proposed to quantitatively characterize the influence of electric field non-uniformity, voltage polarity, gas pressure and SF₆ volume fraction. Some suggestions such as the optimal SF₆ volume fraction of 20%~25% in GIS insulation design are proposed.