Double-circuit transmission lines and even multi circuit transmission lines are trends for development of power system. The paper analyzes the wire layouts of double-circuit line and characteristics of its impedance matrix. And then the sequence currents of double-circuit line are investigated. The paper points out that besides unbalanced net through current, there are distinct circulating currents when double-circuit line is in normal operation or external faults occur. Furthermore, the circulating current is influenced by wire layouts and external faults. Based on the simulation and verification of double-circuit transmission line, the paper calculates the circulating unbalanced degree and investigates the influence of unbalanced circulating current on protection relaying.