A method based on three sequences is proposed for break-conductor fault analysis on double-circuit transmission lines. Considering possible different phenomena of the two circuits in real-world situation， the relationship of the three sequence networks and characteristics of the sequence electrical quantities are studied first. Then combining with the boundary conditions of the disconnected ports， a new approach for calculating the post-fault electric quantities is presented. In order to simplify calculation process and make the characteristics of fault sequence voltages and currents more intuitive， a method of interconnections of sequence networks is presented and drawings of interconnections of sequence networks for typical break conductor fault types are given. The method has a definite physical meaning and is simple in calculation. It can serve as a basic theoretical approach for studying fault transient process and protection action characteristic in open-phase operation state on double-circuit transmission lines.