Abstract: DC gas insulated transmission line（GIL） can be utilized to substitute part of the overhead lines or cables in special environments to improve the flexibility of the selection of the transmission corridor. Key factors that greatly threaten the insulation level of DC GIL include free conducting particles and contents of alumina filler. Influence of the content of alumina filler on the characteristics of flashover and erosion of the alumina filled cast epoxy resin insulator in quasi-uniform field and highly non-uniform field in SF6 gas under negative DC voltage was investigated. Results show that among alumina filled epoxy resin insulators with filler contents from 250 phr to 400 phr, insulator with filler content of 250 phr has most excellent surface dielectric strength than those of other insulators in both short-term flashover and long-term withstand voltage tests, as well as the best erosion property in both the quasi-uniform field and highly non-uniform field. While the insulator with filler content of 350 phr has the worst performance. The flashover takes place in the shallow surface of the insulator according to its microscopic image of the flashover trace. Meanwhile, influence of the filler content on the erosion characteristics of the flashover and the tree properties under DC voltage were studied by analyzing the fractal dimension of tree trace of the flashover.