Biomass pellets are increasingly recognized as a cost-effective and sustainable renewable energy source worldwide. However, comprehensive sustainability assessments of their production processes are scarce. To address this gap, three distinct scenarios in Northeast China were evaluated using emergy, economic, and environmental analysis methods: corn single production, corn–pellet co-production, and pellet production. A modified method for calculating the environmental loading rate (ELR) was proposed, which accounts for the environmental benefits associated with replacing coal with biomass pellets for heating. The results showed that corn–pellet co-production demonstrates superior energy efficiency compared to corn-only production, but presents a contrasting economic profile. The ELR for corn single production and corn–pellet co-production are 1.57 and 1.63, respectively, with corresponding emergy sustainability indices (ESI) of 0.89 and 0.84. After applying the modified method, the ELR and ESI for corn–pellet co-production were adjusted to 0.84 and 1.63, respectively, and the ESI of pellet production increased from 8.24 to 21.15. Furthermore, processing corn straw into biomass pellets for heating can reduce heating costs by approximately USD 254.26/hm² and reduce emissions of SO₂, NO_x, CO, PM_2.5, and CO₂ by 9.12, 19.82, 580.31, 65.86, and 13,060.66 kg/hm², respectively. Sensitivity analysis revealed that transportation distance and renewable electricity have a greater impact on pellet production than corn–pellet co-production. The ESI for pellet production decreases from 21.15 to 14.02 as transport distance increases from 20 km to 100 km, while it rises to 57.81 as the proportion of renewable energy in the power supply increases from 0% to 100%.