It is difficult to accurately determine the ground fault direction using a single-point electrical quantity in a small current grounding system. Thus an improved energy method-based grounding direction criterion is proposed. From the superposition principle, a single-phase ground fault transient equivalent circuit is analyzed, and the zero-sequence energy at the measurement point is composed of the capacitor charging energy at the initial moment of the fault and the steady-state oscillation energy to maintain the LC oscillation. The characteristics of the zero-sequence energy in different grounding modes and resistances are analyzed. To simplify the setting principle, the zero-sequence energy function is discretized and quadratic integration and a practical quadratic integral criterion is proposed. Simulation tests for a metallic fault and a 3000  resistance grounding fault are conducted on the neutral point ungrounded or the neutral point fully compensated ground via the arc suppression coil. Results show that the criterion can distinguish the fault direction in the above fault conditions. This work is supported by the National Key Research and Development Program of China (No. 2017YFB0903100).