Amorphous Silicon oxycarbide (SiCO) shows excellent electrochemical and cycling performance upon lithium intercalation, and is a promising anode material for future lithium-ion batteries. Carbon segregation is a unique molecular structure of SiCO and may plays a key role in its properties, a deep understanding of structure-performance relationship is crutial for reational design of SiCO anode. In this work, first principle calculations were used to investigate the effect of carbon segregation on performance of SiCyO6/5 as anode materials. Based on the calculations results, carbon segregation made small contribution on lithium capacity, while it stablized the whole system by forming three dimensional network, resulting in small volume expansion and stable mechanical properties. The theoretical capacities of SiCO with free carbon were obtained based on the most stable compositions of the lithiated structures, the predicted reversible capacities are comparable to the experimental data. The structure with higher carbon content presents larger Young's modulus during the whole lithiation process, and the saturation points of SiCyO6/5 can also be inferred from the Li content -Young's modulus curves.