The diversity of topologies and the complexity of power supply characteristics make the setting workload of backup protection based on steady-state current large and a mismatch occurs from time to time. Backup protection technology without setting and with an automatic coordination function is the goal of relay protection workers. Based on the analysis of the problems of traditional inverse-time overcurrent (ITOC) protection and the distribution characteristics of positive and negative sequence voltage fault components in a radial distribution network, a backup protection scheme using curve fitting based on system voltage distribution is proposed. The proposed scheme uses the system distribution of compound voltage sequence components and the minimum action time of step-by-step coordination between all protections to fit the action curve with inverse-time characteristics, and obtains the fitted piece-wise function expression. The calculated protection action time can automatically reflect the topological relationship between each protection and fault location, and realize the adaptive coordination of all protections and meet the requirements of selectivity and rapidity. DG access does not change the distribution characteristics of the compound voltage sequence component, so the proposed method is adaptive to the network with DG. Theoretical analysis and simulation results show that the proposed method can automatically realize the fast and step-by-step coordination of upstream and downstream protections in the case of any point fault. This work is supported by the Science and Technology of State Grid Corporation of China (No. 5100-202040327A-0-0-00).