The DC filter is an indispensable equipment in the HVDC transmission system. At present, DC filter protections only use the current protection, and most of which use the tuning frequency current. When the earth fault is close to the head end of filter, there is a greater shift of tuning frequency, and the harmonic current is reduced, so the differential current of filter is reduced, and the protection is lack of sensitivity, and the protection may not operate. In this paper, the harmonic impedance protection is proposed, and the protection makes use of voltage and current, and uses smaller tuning frequency harmonic impedance magnitude as the protection criterion. When the earth fault is close to the head end of filter, the protection with high sensitivity can respond many kinds of faults, but also as a filter detuning monitoring criterion. The simulation using PSCAD verifies the feasibility and reliability of protection, and the harmonic impedance protection will protect the DC filter more reliably.