Since DC transient fault signals lack a fundamental frequency component and line parameters exhibit significant frequency-dependent characteristics, the R-L or Π type line-model based methods struggle to achieve accurate fault location in DC distribution lines. Leveraging the characteristic that voltage is evenly distributed along the healthy line, this paper proposes an accurate fault location method for DC distribution systems by utilizing voltage signals from both ends of the line and two additional measurement points along the line. A distance backup protection scheme based on this fault location method is also designed. The proposed approach applies a least squares algorithm to solve an overdetermined system of equations, thus enhancing the accuracy and reliability of fault location. A three-terminal DC distribution simulation model is built in PSCAD/EMTDC using a frequency-dependent phase-domain line model, and the effectiveness and robustness of the proposed fault location method are verified under different fault positions, fault resistances, noise environments, and positions of the voltage signal.