The invention discloses a quasi-synchronization measuring method based on zero-crossing detection for power transmission line both-end voltage instantaneous voltages, which belongs to the technical field of power system automation. The quasi-synchronization measuring method comprises the steps of: (1) regarding a GPS pulse as a sampling signal of both ends of a wide-area reference clock synchronous power transmission line; (2) regarding the synchronous sampling signal as a reference clock, and utilizing a zero-crossing detection method to obtain initial moments of head and tail ends of the power transmission line at a cycle; (3) and correcting sampling time of the head and tail ends of the power transmission line at the cycle according to the initial moments of the head and tail ends at the cycle. The quasi-synchronization measuring method utilize the zero-crossing points at both ends of the power transmission line as the basis of phase synchronization, has simple implementation method, can effectively solve phase difference between the head and tail ends caused by electromagnetic wave transmission speed in the long-distance power transmission line, and is especially suitable for a superior monitoring control system regarding voltage difference between the head and tail ends of the long-distance power transmission line as source data.