With the aim of developing an improved strategy for the preparation of ethylene-bridged polysilsesquioxanes as thermal insulator materials, this paper describes the synthesis of a crack- and shrinkage-free ethylene-bridged polysilsesquioxane film by the hydrosilylation reaction of hydrodimethyl-silylated oligomethylsilsesquioxane (MSQ-SiH) and dimethylvinyl-silylated oligomethylsilsesquioxane (MSQ-SiVi) in the presence of Karstedt’s catalyst. Polysilsesquioxane precursors were prepared by the sol–gel reaction of triethoxymethylsilane and the successive capping reaction with chlorodimethylsilane and chlorodimethylvinylsilane. The obtained ethylene-bridged polysilsesquioxane film showed lower density and thermal diffusivity (1.13 g/cm³ and 1.15 × 10^–7 m²/s, respectively) than a polymethylsilsesquioxane film (1.34 g/cm³ and 1.36 × 10^–7 m²/s, respectively). As a result of the introduction of the SiCCSi ethylene bridge, the thermal insulation property of the polysilsesquioxane film was enhanced.