Allam cycle is known as oxy-fuel gas-powerd power cycle. A modified Allam cycle co-fired by biomass and natural gas is proposed in this paper, evaluated and optimized. Detailed thermodynamic, economic, and exergoeconomic analyses are reported for the co-fired cycle. And parametric analysis and a tri-optimization are carried out to investigate the effects of cycle variables on the system performance. The results show that as co-firing ratio increases from 20% to 100%, the exergetic efficiency and the levelized cost of electricity vary from 44.3% to 36.8% and 123.2 /MWh to 164.4/MWh, respectively, while the specific negative CO2 emission increases from 44.5 kg/MWh to 251 kg/MWh. The results of tri-objective optimization reveal that the highest exergetic efficiency of 46.85%, lowest levelized cost of electricity of 99.57 $/MWh, and highest specific negative CO2 emission of 323.6 kg/MWh are obtained respectively at different optimal operation conditions.