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Intercropping is shown to have yield advantages over sole cropping, but it is unknown how much yield increase is due to belowground interspecies interactions. This study determined land equivalent ratio (LER) and water use efficiency (WUE) of spring wheat (Triticum aestivum L.)/maize (Zea mays L.) intercropping and quantified the magnitude of yield increases due to belowground interspecies interactions. Field experiments, conducted at Wuwei (37 degrees 96’N, 102 degrees 64’E) in 2008 to 2010, included sole wheat (W), sole maize (M), wheat/maize intercropping with no root-barrier (W/M), with a plastic sheet barrier (PW/M), or with a nylon mesh barrier (NW/M), vertically placed to one meter deep between the intercrops. Wheat/maize intercropping increased grain yields by 46% in 2008, 26% in 2009, and 64% in 2010, and improved WUE by 49, 30, and 20% in the respective years, compared with the corresponding sole cropping. The LER values ranged from 1.24 to 1.60 for W/M, suggesting that the grain yield of intercropping per hectare is equivalent to the yield that sole wheat or sole maize would produce on 1.24 and 1.60 ha. The W/M system had total yield of 10.6, 11.1, and 16.9 t ha(-1) in 2008, 2009, and 2010, respectively; they were 20, 14, and 15% greater compared with the PW/M system; belowground interspecies interactions contributed 32% of the increased yield in 2008, 29% in 2009, and 40% in 2010. With the large, positive belowground interspecies interactions, wheat/maize intercropping is shown to be a promising farming practice for improving crop productivity and WUE in arid irrigation areas.