A mathematical model for ore reduction mainly by H₂ in shaft furnace was established by a set of conservation equations of materials and heat, and other constraint conditions. All consumptions and productions could be calculated if the composition of N₂, the value of V(CO)/V(H₂), the metallization ratio of DRI, the temperature of input gas, etc. were given. It shows that the input gas can be reduced from 1650 N·³ for H₂ reduction to 1200 N·m³ by CO addition. When the volume ration of CO to H₂ V(CO)/V(H₂)was 0.6, the energy utilization ratio of shaft furnace was maximum without N₂ addition; when V(CO)/V(H₂)=0.3, the optimum composition of N₂ was 11%; when V(CO)/V(H₂)=0, the optimum composition of N₂ was 25%. The N₂ addition may result in H₂ consumption, but could not reduce the total input gas. The simulation will provide reference to process design, operation and energy saving.