The invention relates to a half-wavelength transmission line quick underreaching protection method. The method comprises steps: a phase current difference fault component starting element is built; a time different criterion is used to lock fault outside a forward region; a direction element criterion is used to lock fault outside a reverse region; and when the starting element criterion and the direction element criterion are met, protection of the transmission line at the side is not locked, and the outlet is protected. The technical scheme of the invention is non-unit protection, the fault can be quickly recognized, an action is made reliably, and the direction element criterion and the starting time difference criterion can be ensured to have enough sensitivity.