Blast furnace operation with natural gas (NG) injection is one of the effective measures to save energy, reduce CO2 emission, and decrease environmental load for iron and steel industry. Numerical simulations on blast furnace operation with NG injection through tuyeres are performed in this paper by raceway mathematical model, multi-fluid blast furnace model, and exergy analytical model. With increasing NG injection volume, the simulation results are shown as follows: (1) the theoretical flame temperature and bosh gas volume can be constant by decreasing blast volume and increasing oxygen enrichment. (2) The utilization rate of CO enhances while that of H2 decreases. The proportion of H2 in indirect reduction tends to be increased, which accelerates the reduction of burdens. The pressure drop shows that the permeability of blast furnace gets better. The blast furnace productivity is increased from 2.07 to 3.08 t · m−3 · day−1. The silicon content in hot metal is decreased from 0.26% to 0.05%. When BF operation with 125.4 kg · tHM−1 NG injection, coke rate and carbon emission rate are decreased by 27.2% and 32.2%, respectively. (3) The thermodynamic perfection degree is increased from 88.40% to 90.50%, the exergy efficiency is decreased from 51.94% to 49.02% and the chemical exergy of top gas is increased from 4.69 to 6.22 GJ · tHM−1. It is important to strengthen the recycling of top gas.