This paper analyzes a refusal-operation of line differential protection in high resistance grounding fault, and works out the reason of no simultaneous operation of differential protection starting element in both sides of line is lack of sensitivity of current starting element in weak fault side. Therefore, an improved auxiliary start-up method of differential protection double-ends zero sequence current and voltage is presented. This new method improves the starting sensitivity of current differential protection in weak fault side through fault component information of current or voltage in both sides of line. Fault recording data replaying and RTDS simulation results show that the protection performance of improved current differential protection is apparently enhanced in high resistance fault, especially in the one-side fault and the side far from the fault only feels the tiny fault component.