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In semi-arid areas on the Western Loess Plateau, China, crop growth has been greatly limited by water supply. The comparative wet season in the region runs from July through September, and about 60 % of the yearly precipitation total falls within this period. Rainfall is very sparse from November of the preceding year through June of the following year. Water harvesting agriculture has been shown to be one of the most efficient ways to increase water use efficiency (WUE) and crop productivity for wheat, corn, broccoli etc. however, research on WUE in potato (Solanum tuberosum L.), one of the dominant crops in the area, is sparse. Field experiments on supplemental irrigation (amount and timing) using catchment rainfall were carried out in both 2008 and 2009 in Dingxi, a typical potato growing area, for the purpose of determining effective utilization of the limited supplemental irrigation water for potatoes. Soil water storage, evapotranspiration (ET), WUE, irrigation water use efficiency (IWUE), tuber yield and size and starch and protein contents were determined in this research. The results showed that both ET and tuber yield increased with supplemental irrigation, consequently WUE was improved. IWUE appeared to be better with 45 mm of irrigation applied at the vegetative growth stage and tuber yield was increased by 7-18 % compared to the treatment without irrigation (T1), and WUE and IWUE were significantly higher than other treatments in both years. Compared to the control, WUE with 45 mm of irrigation at the vegetative growth stage increased by 1.28 % and 2.17 % in 2008 and 2009, respectively. In addition, 45 mm of irrigation at the vegetative growth stage also increased the percentage of large tubers, and the sum of large and medium tubers. Tubers under such practice were of acceptable size and had the highest protein content. Therefore, we conclude, 45 mm of supplemental irrigation at the vegetative growth stage is the best way to use the limited available supplemental irrigation water from catchment rainfall for rain-fed potatoes grown in semi-arid areas on the Western Loess Plateau of China. Applying more water, or at a later growth stage of potato, was less effective.