In order to reduce the vessel′s shaft-related electric field, the equivalent circuit of shaft-related electric field was established on the basis of the analysis of the ship aft end structure, and it proved that the fluctuating resistance between the carbon brush and slip ring is the main factor that produces shaft-related electric field. The principle of the ASG (active shaft grounding) system was studied, and the prototype ASG system was developed and tested by shrinkage ratio ship model and real vessel experiments. The results showed that the shaft-rate electric field can be reduced effectively by 90% when the ASG system is employed. In addition, the ASG system will make static electric field increase when the resistance between the carbon brush and slip ring counts much in the equivalent circuit. After analyzing the factors that affect the output current of ASG system, it was found that the maximal output current is firmly related with the cathodic protection state, but not the sailing speed.