Abstract: Ranging accuracy is a key factor affecting the successful connection between relief wells and accident wells. In order to improve the measurement accuracy of the active magnetic ranging system based on injected current, the electric field distribution in each medium was analyzed, and the current density distribution model of the accident well casing was established. By analyzing the current density distribution law of accident well casing, the calculation model of the optimal distance between the electrode and the probe based on the principle of the maximum magnetic induction intensity at the measuring point was established and the effectiveness of the model was verified by comparison test results. Calculations with examples show that the optimal distance between the electrode and the probe is related to not only the distance between the electrode and the accident well but also the relative well inclination angle. In addition, it has nothing to do with the injected current intensity. The distance between the peak point of the casing current density of the accident well and the coordinate point of the accident well is approximately equal to that between the electrode and the probe in the accident well. The optimal distance between the electrode and the probe is approximately equal to the product of the distance between the electrode and the accident well and the cosecant of the relative inclination angle. According to the calculation results of the optimal distance, a reasonable design of the distance between the electrode and the probe can contribute to the improvement measurement accuracy of the active magnetic ranging system based on injected current.