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. The calculation method of the optimal distance from the electrode to the probe tube based on the principle of the maximum magnetic induction intensity at the measuring point. Calculations with examples show that the optimal distance from the electrode to the probe pipe is not only related to the distance from the electrode to the accident well, but also to the relative inclination angle, but has nothing to do with the injected current intensity; The distance from the well coordinate point is approximately equal to the distance from the electrode to the probe tube of the accident well; the optimal distance from the electrode to the probe tube is approximately equal to the product of the distance from the electrode to the accident well and the cosecant of the relative inclination angle. According to the calculation results of the optimal distance, reasonable design of the distance from the electrode to the probe is beneficial to improve the accuracy and range of ranging.