The anthropogenic CO2 emissions have been proven to cause climate change in recent decades. To reduce the CO2 emissions in energy industry, a modified Allam cycle co-fired by biomass and natural gas is proposed and evaluated. Comprehensive thermodynamic and mathematical models of the system are developed. A sensitivity analysis is conducted to investigate the effects of co-firing ratio as well as other key parameters on the system performance. The results show that the introduction of natural gas improves cycle performance. The energetic efficiency of the proposed system is in the range of 43.44% to 49.28%. With the increase in the co-firing ratio from 20% to 100%, the energetic efficiency and exergetic efficiency decrease continuously while the specific negative CO2 emission increases from 42.6 kg/MWh to 241.8 kg/MWh. The effect of the outlet temperature of the combustor on the energetic efficiency is also investigated. It shows that increasing the outlet temperature of the combustor has a positive impact on the energetic efficiency of the overall system.