When a single-phase ground fault in a neutral point ungrounded system occurs, the fault current includes not only a power frequency component, but also a transient component with short duration, large original amplitude, and relatively concentrated energy. Rapid and accurate calculation of the full fault current is the basis for realizing full fault current compensation. One method for full fault current calculation of a single-phase grounding fault in a non-solidly grounded system is proposed. Here a Γ equivalent circuit is applied to model the feeders, and its feasibility and rationality are analyzed. Then based on the Kirchhoff Current Law and three-phase time-domain voltage signals from bus TVs, the full fault current can be calculated via differential equations that are used to describe the Γ equivalent circuit. The calculation results can provide a basis for full fault current compensation. A simulation case indicates that the method used for calculating a full fault current based on a Γ equivalent circuit has the advantages of discernible principle, rapid and accurate calculation, less electrical quantities needed, and being independent of fault line selection. It can meet the demand of a full fault current compensation for a single-phase grounding fault in a non-solidly grounded system.