The purpose of this study is to optimize the mechanical properties of insulation materials for dry-type transformers. In order to improve the mechanical properties of epoxy resin composites, inorganic rigid particles were added by blending. Epoxy resin materials with mass fractions of stable $\boldsymbol{\alpha}- \mathbf{Al_{2}O_{3}}$ content of 0%, 1%, 2%, 3%, 4%, and 5% were prepared through blending, ultrasonic dispersion, heating stirring, and vulcanization molding steps. Tensile and impact tests were carried out according to standards. The test results showed that the mechanical properties of $\boldsymbol{\alpha}-\mathbf{Al_{2}O_{3}}$ composites first improved and then deteriorated as the mass fraction of $\boldsymbol{\alpha}-\mathbf{Al_{2}O_{3}}$ increased. The composite material with a mass fraction of 4% $\boldsymbol{\alpha}-\mathbf{Al_{2}O_{3}}$ had the optimal mechanical properties, with a 24.3% increase in tensile strength and a 24.4% increase in impact strength compared to pure epoxy resin. This conclusion will help to prepare insulation materials for dry-type transformers with better mechanical properties and ensure the safe and stable operation of transformers.