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Water harvesting is a core technology for collecting fresh water in arid areas. In this study, we design a three-dimensional cactus stem-inspired water harvesting system (WHS) with directional transport of absorbed fog. The bioinspired WHS consists of two distinct functions. One is an effective water-absorbing function with an antievaporating feature, and the other is an on-demand water-releasing function. The excellent water absorption capability of a mucilage-filled cactus stem covered with a cuticle is mimicked by a cylindrical double structural system (DS) comprising an interpenetrating polymer network (IPN) hydrogel with good water retention capacity and a superhydrophobic copper mesh (SHPM) that prevents the re-evaporation of absorbed water. DS harvests water at a rate of 209 mg cm–2 h–1 and exhibits enhanced water collecting performance, i.e., 1.2, 1.3, and 2 times higher than that of the superhydrophilic IPN hydrogel, SHPM, and pristine copper mesh (PTM), respectively. The detailed fog harvesting mechanism of DS is examined through an X-ray imaging technique, and the water harvesting mechanism is described in terms of volumetric expansion of IPN hydrogel, absorption of microdroplets on mesh humps, and thickness of the water-film between mesh fibers. In addition, the function of water release is demonstrated with the aid of the thermoresponsive property of the IPN hydrogel. This biomimetic WHS may aid in developing effective three-dimensional plant-inspired fog collectors.