An automotive Relay model-based new algorithm is presented to quickly search the transmission section. Firstly, the power network can be turned into a topology, with nodes seen as stations and line impedance as path length. Secondly, according to certain rules, several cars of the same type take part in a relay between originating and terminal station. Thirdly, as fuel consumption is proportional to the path length, cars could be sorted according to its remaining amount of gasoline, which means K shortest paths could be searched through backtracking. Large amounts of sub graphs don’t need deriving, and the shortest paths could be got without repeatedly solving. The DC power flow transferring coefficient is introduced, and the coefficients of each branch in the paths searched are calculated. And the branch whose coefficient above certain value should join the transmission section, an accurate transmission section set without leaking lines is reached. Compared to the usage of a single path whose search range is too small, the automotive relay algorithm could avoid leaking lines and ensure integrity of transmission section. Finally an IEEE30 node system simulation is analyzed, demonstrating the effectiveness of the algorithm.