To solve the vulnerability of current differential relay protection to the negative impact of the distributed capacitive current in the long UHV transmission line， a principle of and a criterion for current differential relay protection based on the Marti model are proposed. Different from the Bergeron model differential protection， the frequency-dependent characteristic of line parameters is taken into account in this principle so that the calculating result is more accurate. The simulated results concerning the first UHVAC transmission line in the country show significant improvement in both the reliability and sensibility of the principle proposed.