A method for accurately testing yarn hairiness through stretching one-directionally, belonging to a technical field of textile testing, is provided. A suction pipe is provided on an external side of a laser and connected with an exhausting fan through a sleeve connector and a hose. An air inlet of the suction pipe generates a negative pressure under an effect of the exhausting fan to laterally suck the yarn running at a tension state among the laser, a projection receiver and the suction pipe. The hairiness on a yarn surface stretches straight one-directionally towards an airflow direction at a height of a center of the air inlet of the suction pipe. The one-directional straight stretched hairiness on the yarn surface is accurately projected to the projection receiver of a testing head at an equal length for accurately testing a hair amount and a hair length.