Mixed anhydride is an effective method to regulate the crosslinked structure of epoxy resins, which is expected to achieve an improvement in the dielectric properties of the epoxy curing samples and thus improve the insulation performance of the insulation core of the UHV bushing. In this paper, five epoxy resin samples with different acid anhydride curing agent ratios are prepared by hot pressing method. The thermal and dielectric properties of the samples are tested, the best anhydride curing agent ratios were selected, and the steric effects caused by methyl substitution are analyzed. The results show that the mixed anhydride curing sample with the molar ratio of methylhexahydrophthalic/hexahydrophthalic anhydride of 8:2 reaches the highest glass transition temperature and the electrical strength is improved with essentially constant dielectric loss, which is closely related to the increased rigidity of the system.