Currently, sustainable utilization, including recycling and valorization, is becoming increasingly popular in waste management. Black soldier fly larvae (BSFL) can convert the carbon (C) and nitrogen (N) from organic waste into biomass and improve properties of the substrate to reduce greenhouse gas and NH3 emissions. In this study, the recycling of C and N and the emissions of greenhouse gas and NH3 during BSFL bio-treatment of mixtures of pig manure and corncob were investigated under different C/N ratios. The results indicated that initial C/N ratios of feedstock are a crucial parameter affecting the biomass generation of larvae. The BSFL recycled approximately 4.17–6.61% of C and 17.45–23.73% of N from raw materials under different C/N ratios. Cumulative CO2, CH4, NH3, and N2O emissions at the different C/N ratios ranging from 15 to 35 were 107.92–151.68, 0.08–0.76, 0.14–1.17, and 0.91–1.18 mg kg−1, respectively. Compared with conventional composting, BSFL treatment could reduce the total greenhouse gas emissions by over 90%. The study showed that bio-treatment of mixtures of pig manure and corncob with a proper C/N ratio by BSFL could become an avenue to achieve higher nutrient recycling, which is an eco-friendly process.