3-Indoleacetic acid is a kind of plant auxin. Auxin has many physiological effects, which are related to its concentration. Low concentration can promote growth, and high concentration will inhibit growth even kill plants. This inhibitory effect is related to whether it can induce the formation of ethylene. The physiological effects of auxin are manifested on two levels. At the cellular level, auxin can stimulate the division of cambium cells; stimulate the elongation of branches and inhibit the growth of root cells; promote the differentiation of xylem and phloem cells, promote cuttings root, and regulate the morphology of callus. At the organ and whole plant level, auxin works from seedling to fruit ripening. Auxin controls the reversible red light inhibition of hypocotyl elongation in seedlings; when it transfers to the underside of the shoots, it produces branch geotropism; when it transfers to the backlight side of the shoots, it produces branch phototropism; 3-Indoleacetic acid causes apical advantage and delays leaf senescence. Auxin promotes flowering, induces parthenocarpic fruit development, and delays fruit maturity.