The present invention relates to the technical field of relay protection and particularly relates to a longitudinal zero sequence direction protection method of a one-tower double-circuit parallel transmission line. The method comprises a step of collecting the electrical quantity of two ends of a transmission line when the transmission line has a fault, a step of judging whether one end of the transmission line has negative sequence current or not, a step of judging whether the zero sequence direction of one end of the transmission line is positive or not if the one end of the transmission line has the negative sequence current, a step of judging whether the zero sequence directions of two ends of the transmission line is positive or not if the zero sequence direction of one end of the transmission line is positive, a step of judging that the fault is an internal region fault, and acting longitudinal zero sequence protection, otherwise, judging that the fault is an external region fault, and not acting longitudinal zero sequence protection. Through employing different zero sequence direction judgment methods for the different fault types in the transmission line, the accuracy of the longitudinal zero sequence direction protection is improved, the problem that the longitudinal zero sequence direction protection of the other non-failed line acts wrongly if one transmission line is failed in the one-tower double-circuit parallel transmission line is solved, and the reliability and the security of the transmission line are improved.