Abstract: Alfalfa (Medicago sativa) has high nutritional quality and biomass production and is widely used as pasture in animal production and soil erosion control. There are many factors influencing the production of alfalfa, including fertilizer and water use. The applications of both nitrogen and irrigation have been the critical factors for improving water use efficiency without considerable yield reduction in alfalfa. Thus the determination of a reasonable application of nitrogen and irrigation is important for the optimization of biomass distribution characteristics and improvement of water use efficiency in alfalfa fields in Hexi Corridor. To that end, a field study was conducted in 2014 in Jiuquan City (in the Hexi Corridor of Gansu Province, China) to research the effects of different irrigation amounts (W₁: 60% of the conventional irrigation amount; W₂: 80% of the conventional irrigation amount; W₃: the conventional irrigation amount, 1 920 m³·hm^-2) and nitrogen application rates N₁: 0 kg(N)·hm^-2; N₂: 40 kg(N)·hm^-2; N₃: 80 kg(N)·hm^-2; N₄: 120 kg(N)·hm^-2 on biomass distribution characteristics and water use efficiency of 2-year alfalfa plantations. The study investigated the effects of nitrogen application, irrigation and the related interaction on alfalfa plant height, branch number per plant, leaf-stem ratio, root volume, root biomass, aboveground- belowground biomass ratio and water use efficiency. The results suggested that W₂ and W₃ treatments significantly increased plant height, branch number per plant and aboveground biomass. It also increased root biomass, root volume in 2040 cm, 4060 cm and 060 cm soil layer and water use efficiency. Moreover, plant height, branch number per plant and aboveground biomass were not obviously different between W₂ and W₃ treatments. This suggested that 80% of the conventional irrigation scheme not only maintained normal biomass production in alfalfa plants, but also improved water use efficiency. Branch number per plant, leaf-stem ratio, root volume, root biomass, aboveground-belowground biomass ratio and water use efficiency initially increased and then decreased with increasing nitrogen application. These parameters peaked under the treatment of 80 kg(N)·hm^-2 nitrogen, which suggested that nitrogen application significantly influenced root development and water use efficiency of alfalfa plants. Besides this, W₂N₂ or W₂N₃ treatments had the potential to optimize plant height, branch number per plant, root volume and belowground biomass in the 020 cm, 2040 cm and 060 cm soil layers. The ratio of aboveground to belowground biomass and water use efficiency was also optimized under W₂N₂ or W₂N₃ treatments. It was concluded that 80% of the conventional irrigation amount and 80 kg(N)·hm^-2 of nitrogen application were the optimal treatment for alfalfa pasture in Hexi Corridor.