Abstract: Although water deficit and soil phosphorus (P) deficiency are key limiting factors of maize production, it still remains unclear how water and P regulate maize root morphology and P uptake. In this study, a pot experiment was carried out to explore the coupled effects of water and P on maize root growth and P uptake under four water gradients 35% (W1), 55% (W2), 75% (W3), 100% (W4) of field capacity and two P treatments high P of 205 mg(P).kg^-1(soil) and low P of 11 mg(P).kg^-1(soil). The results showed that irrespective of P supply, shoot dry weight (SDW), root dry weight (RDW), total root length (TRL) and root surface area (RSA) initially increased and then decreased with increasing water supply. This trend was similar to trend in soil available P content. Root mass ratio (RMR) and average root diameter (ARD) declined, while shoot P content and accumulation steadily increased with increasing water supply. There were both adverse effects of water deficit and excess water supply on root growth and dry matter accumulation of maize. While soil P acquisition was inhibited by water deficit (W1), it was greatly improved by excess water supply (W4). Slight water stress (W2) increased maize root growth and dry matter accumulation, but decrease excess soil P uptake. Adequate water supply (W3) simultaneously improved maize root growth, dry matter accumulation and soil P uptake. P application obviously increased SDW, RDW (except for in W4 treatment), TRL, RSA, P content (except for in W4) and total P accumulation in plant, but decreased RMR. Two-way analysis of variance showed that the relative contribution of water to SDW, RDW, RMR, TRL, RSA, ARD, shoot P content, shoot P accumulation and soil available P content were 45.94%, 36.71%, 67.95%, 59.63%, 58.34%, 81.86%, 24.75%, 35.66% and 3.00%, respectively. The relative contributions of P to the above parameters were 34.78%, 21.19%, 14.84%, 9.22%, 9.21%, 1.56%, 35.54%, 49.75% and 94.40%, respectively. It was clear that water was more important for the regulation of maize root morphology and dry matter accumulation, and P was more important for the regulation of P absorption in maize aerial parts and soil available P content. In all, the acquisition of soil P by maize root was connected with root morphology-oriented adaptations under low P, but was connected with root physiology-oriented adaptations under high P. An appropriate coupling of water and P improved root growth and dry matter accumulation, and decreased excess soil P uptake by maize root.