Abstract: On January 14, 2018, the MMS4 satellite located outside the Earth’s bow shock observed an upstream energetic ion event, during which energetic ions moved from the bow shock along the interplanetary magnetic field toward the Sun. This study analyzes the changes in the Earth’s polar magnetic field during the upstream ion event using the ionospheric equivalent current method. The conclusions are as follows: Before the occurrence of the upstream energetic ion event, the polar magnetic field exhibited intense disturbances, with a delay of several hours between the two. Prior to the event, the ionospheric current disturbances showed a significantly enhanced negative current vortex on the dayside, with curl-free component currents flowing from the dayside to the nightside. The active ionospheric currents in the polar region provided energy for the acceleration of oxygen ions, transforming them into energetic oxygen ions. These energetic oxygen ions subsequently outflowed from the magnetosphere, reached the bow shock, and finally moved upward along the interplanetary magnetic field. Upstream energetic ions may impact future manned space activities in outer space. This study provides insights for the forecasting of future upstream ion events.