Abstract: Diverse materials synergistic co-composting has been the prevailing trend for sustainable agriculture in recent years, according to the diverse types and complementary properties of organic wastes. Organic wastes can be used as composting raw materials. When converting to biochar after pyrolysis, the organic wastes can serve as external additives to regulate the maturity and humification during composting. Therefore, this study aims to explore the effect of biochar from three sources on the maturity and humification during diverse materials synergistic composting. The pig manure, kitchen waste, and corn straw were taken as the raw materials. At the same time, pig manure, kitchen waste, and stover biochar were transformed after high-temperature pyrolysis and then added to the pile. A systematic investigation was also implemented to clarify the effects of three biochars from different sources on the maturity and humification during the co-composting. The research results were as follows: Three types of biochar promoted the temperature and the compost maturity during composting. Among them, there was a significant increase in the maturity of the compost treated with pig manure biochar and stover biochar. The seed-germination index (GI) of the compost products was 23.42% and 16.84% higher than that of the control group, respectively. Meanwhile, both stover and kitchen waste biochar promoted the humification of composting. The ratio of humic acid to fulvic acid (HA/FA) of the compost products was 67.31% and 65.38% higher than that of the control group, respectively. While the treatment with pig manure biochar was slightly lower than that of the control treatment. Therefore, the treatment with the stover biochar simultaneously enhanced the compost maturity and the degree of humification. The correlation analysis indicated that the stover biochar improved the compost maturity by reducing the electrical conductivity (EC) and the content of ammonium nitrogen (NH4+-N) of the compost, and then promoting the formation of humus in the compost by regulating nitrogen transformation, thereby enhancing the degree of compost humification. This property was attributed to the large specific surface area and abundant pore structure of stover biochar. In conclusion, the multi-material co-composting with the corn stover biochar can be expected to simultaneously enhance the compost maturity and the degree of humification.