According to the many types of single line faults and cross-line faults of four jointed parallel lines on the same tower， twelve sequence components separated each other can be obtained by the step matrix transform to the twelve phase voltage and current of four jointed parallel lines on the same tower， so as to decouple the mutual effect. Based on the analysis of the features of each component， the relevant sequence networks are set up. Using twelve sequence network and borderline condition， all sequence voltage and current of any branch can be obtained， then the twelve phase voltage and current of the branch can be calculated by using matrix transform. The value of short circuit current by simulation is compared to that by calculation. The simulation results indicate that the short circuit current calculated by twelve sequence components is correct.