The invention relates to a low-pass digital filtering method applicable to direct current line protection. The low-pass digital filtering method is characterized by comprising the following steps of 1, searching and selecting a low-pass filter in Keyser window design as a low-pass digital filter for the direct current line protection; 2, in a Keyser window function, enabling to be equal to 7.865, and N to be equal to 21, and working out the coefficient of the Keyser window low-pass digital filter; 3, enabling a sampling value sequence of number x(n) of direct current line protection to be multiplied by a (n) sequence of number and then making summation of the two parts to work out the output sequence of number of the Keyser window low-pass digital filter after filtering; and 4, transmitting a sequence of number y(n) obtained by filtering to a direct current line protection program to perform calculation of protection criteria. By virtue of the low-pass digital filtering method, direct current components in voltage and current can be accurately extracted from sampling signals comprising direct current component, transient state high frequency oscillation component and noise, so that correct measurement quantity is provided for backup protection of the direct current line, thereby ensuring selectivity of backup protection actions of a direct current power transmission line.