Four-parallel transmission lines reduce the required space of right-of way of transmission. However， technical difficulties，such as fault location and relay protection， occur in practice. Twelve-sequence component method is introduced to decouple mutual electromagnetic effect between four-parallel transmission lines. The twelve-sequence components are independent from each other. The characteristics of the twelve-sequence components are studied and the related sequence networks are introduced. The twelve-sequence component method can not only be the basic method of the fault analysis but also the basic theory of the fault location and relay protection for the four-parallel lines. Large amount of simulation results of EMTP simulation demonstrate that the twelve-sequence component method is correct and valid for fault analysis.