Abstract: As an important link to ensure the grid connection of offshore wind farms, the distributed capacitance of the submarine cable is much higher than that of overhead lines, so the generated capacitive current cannot be ignored. The existing current differential protection criterion does not consider the distributed capacitance and is affected by the load current, besides, the anti-transition resistance ability is poor. Therefore, it is not highly applicable on ultra-long submarine cable transmission lines. In allusion to the defects in existing protection criterions, an improved current differential protection criterion was proposed, in the improved criterion the larger distributed capacitance current in the transmission line was compensated, so the protection malfunction during both normal operation and external short circuit was resolved, and the impact of load current on the action characteristic of differential protection was reduced, thus, the ability of anti-transition resistance was improved. A 220kV long distance cable line model was built on PSCAD/EMTDC platform to conduct the simulation analysis on the proposed improved protection scheme, simulation results show that on the basis of ensuring inaction under external faults the proposed new current differential protection criterion for ultra long submarine cable significantly improves the ability to resist transition resistance of faults in the area, so it is an ideal current differential protection scheme for the long high-voltage cable line.