The invention provides a halogen-free medium-Tg high-CTI flame-retardant epoxy glass fabric base laminate and a preparation method thereof. The preparation method comprises following steps: step 101,filler dispersion, wherein 48-52 parts by weight of a phosphorus-containing epoxy resin and 18-22 parts by weight of novolac epoxy resin are taken, 48-52 parts by weight of PMA is added into a mixtureof the phosphorus-containing epoxy resin and the novolac epoxy resin, 118-122 parts by weight of a filler is added while stirring is conducted, and then stirring is accelerated; the filler is aluminum hydroxide and/or silicon dioxide; step 102, 178-182 parts of novolac epoxy resin and a dispersed filler obtained in the step 101 are mixed to obtain a mixture; step 103, 28-32 parts of phenolic resin, 88-92 parts of benzoxazine resin, 178-182 parts of a phosphorus-containing flame-retardant curing agent and 0.45-0.55 part of catalyst dimethylimidazole are added into the mixture obtained in the step 102; 104, S/G measurement after machine shutting down; step 2, impregnation; step 3, stacking; step 4, combining; step 5, pressing; and step 6, cutting. According to the preparation method, the ratio of common epoxy resin, the phosphorus-containing epoxy resin, benzoxazine resin, aluminum hydroxide and other raw materials is optimized, so that the high CTI function is achieved.