Water and fertilizer are major factors affecting maize (Zea mays L.) yield on the Loess Plateau of China. To determine the effects of zinc (Zn) fertilizer on maize yield and water-use efficiency (WUE) under different soil water conditions, a field experiment was conducted using cultivar Zhengdan 958. The effects of Zn (0, 20, 50, and 80 kg ha−1 ZnSO4) on maize transpiration, leaf relative water content (RWC), root morphological characteristics, root hydraulic conductance (RHC), yield and WUE under rainfed conditions (well-watered, WW) or rainout shelter conditions (drought stress, DS) were assessed. Compared with the zero-Zn controls under the respect water treatments, the chlorophyll content, stem sap flow rate (SFR) and RHC in response to Zn increased by 8%, 30% and 177%, respectively, at 20 kg ha−1 of Zn under WW and 18%, 46% and 52%, respectively, at 50 kg ha−1 under DS. Compared with those in the controls, leaf RWC in response to 50 kg ha−1 Zn under DS and 20 kg ha−1 under WW increased significantly. Zn fertilizer increased grain yield (GY) by 12.5% and 7.5% and WUE by 11% and 6.5% at 50 kg ha−1 under DS and 20 kg ha−1 under WW, respectively, relative to the controls without Zn. These results suggest that in the alkaline soil area of Loess Plateau, 50 kg ha−1 Zn can effectively improve water absorption capacity of maize, thus increasing yield and WUE under DS, while 20 kg ha−1 is sufficient under WW.