A composite with ultrafine SiO_x (x = 1.57, around 2 nm) nanoparticles confined in a carbon framework is synthesized by a simple thermopolymerization process and subsequent heat treatment. In the composite, the carbon framework can provide a consecutive network to improve the electrical conductivity of the composite and cushion the volume expansion to prevent the active material peeling from the current collector. The ultrafine SiO_x nanoparticles can alleviate mechanical strain and shorten the diffusion/transport distance of lithium ions and electrons. In consequence, the as-synthesized composite delivers a high reversible capacity of 540 mA h g⁻¹ at a current density of 500 mA g⁻¹ after 200 cycles. The composite delivers good electrochemical performance, making it a promising candidate for the next-generation high-energy LIBs.