The invention discloses a transverse differential protection method for a DC power transmission line based on a DC filter branch current, and the method comprises the steps: 1, measuring current instantaneous values, passing through an I-pole DC filter branch circuit and an II-pole DC filter branch circuit, of a rectification side of a DC power transmission system, and recording the current instantaneous values as i(t) and i<II>(t); 2, calculating a transverse differential protection current instantaneous value according to a formula: i<hc>(t) = i(t) + i<II>(t), taking 10 ms as a data window, calculating a transverse differential current, the root-mean-square value of the I-pole DC filter branch circuit and the root-mean-square value of the II-pole DC filter branch circuit, and respectively recording the transverse differential current, the root-mean-square value of the I-pole DC filter branch circuit and the root-mean-square value of the II-pole DC filter branch circuit as I<hc>, I, and I<II>; 3, carrying out the fault discrimination: start timing from zero when I<hc> is greater than I<set>, and recording a timing result as t; setting a mark flag = 1 if t is greater than t<set>, judging that a DC power transmission line region has a fault at this moment, and executing step 4, wherein I<set> is a current operation set value, and t<set> is a time delay set value; 4, carrying out the discrimination of a fault region: judging that the fault happens in a I-pole line region if I / I<II> is greater than one, or else, judging that the fault happens in an II-pole line region.