UHV long transmission line is of its distinct characteristics of distributed parameters and large capacitive currents. The analysis shows that the measured impedance at the head of the line is not proportional to the fault distance when a three-phase fault occurs at the end of the line. Studies indicate that the relationship between the measured impedance and fault distance for phase-to-phase fault is a hyperbolic tangential function. The traditional distance protection for grounding faults has inherent defect when used in UHV long transmission line. This paper analyzes and deduces a novel calculation formula of distance protection for grounding faults， and proposes an improved method for distance protection. Simulation results have confirmed the validity and reliability of the proposed method in UHV long transmission line.