Silicon oxycarbide (SiOC) materials are considered as a promising high capacity anode for next-generation Li-ion batteries. However, the rapid capacity fading and poor cycling stability often occur in the SiOC materials with high Si content. In this work, an effective and facile strategy is reported to construct nanocomposites of MnO/SiOC, in which SiOC acts as buffer agent and conductive matrix, while MnO contributes to the major reversible capability. Microalgae (Chlorella) serves as both biological template and carbon source to synthesize SiOC microspheres with the assistance of supercritical CO2 fluid, then rice-like MnO nanoparticles are tightly embedded in the SiOC matrix through the heavy metal ion bio-sorption behavior of Chlorella. As anode materials for Li-ion batteries, the C/MnO/SiOC composites exhibit high reversible capacity of 770 mAh g(-1) after 200 cycles at a current density of 100 mA g(-1) with excellent cycling stability and great rate capability. This work provides a novel strategy to fabricate SiOC-based high capacity anode materials for advanced Li-ion batteries.