Reformed coke oven gas (RCOG), a hydrogen-rich gas, is a promising auxiliary fuel for ironmaking blast furnaces (BFs) for lowing the coke consumption and production cost. It can be practiced via shaft injection. However, the effect of RCOG shaft injection on the inner state and BF performance has not been fully understood. In this research, the injection of RCOG at the BF shaft is investigated using the multiphase BF model, considering the respective burden layer structure. The in-furnace phenomena with and without RCOG shaft injection are first compared. The simulation results show that the BF inner state can be affected by the thermal energy and the reductive agents brought in by the RCOG and the interaction between the RCOG flow and the gas flow from the BF tuyere level. Then, the effect of different injection rates of RCOG and injection angles on the BF inner state and production performance are studied. It is indicated that with the increased RCOG injection rate, the hydrogen concentration increases, and the near-wall ferrous oxide reduction degree can be improved; the profiles of the cohesive zone (CZ) along the BF center moves upward, and the profiles of CZ becomes narrower. Also, the coke rate decreases as the shaft injection rate increases. On the other hand, the injection angle has an insignificant effect on the BF inner situation and reductant utilization efficiency under the simulation conditions. Overall, the BF RCOG shaft injection can make a positive contribution to coke saving and BF performance improvement.