To determine the optimum furrow-to-ridge ratio and a suitable ridge-mulching material for alfalfa production in a ridge-furrow rainwater harvesting system in the semi-arid Loess Plateau, a field experiment with a randomized complete block design was conducted during the 2012 and 2013 alfalfa growing seasons. The experiment was designed to investigate the effects of different mulching materials (manually compacted soil crust, biodegradable mulch film, and common plastic film) and furrow to ridge ratios (60:30, 60:45, and 60:60[cm:cm]) on soil moisture and alfalfa forage yield, with traditional flat planting (TFP) as a control. The average number of ineffective rainfall events (53) was greater than that of effective rainfall events (27) over these two years, and the contribution rate (19%) of the ineffective rainfall to annual rainfall was less than that (81%) of the effective rainfall. Compared with TFP, increases in the soil water storage at depths of 0-140 cm were 12.8, 19.2, 24.4, 26.0, 30.7, 40.5, 29.9, 37.1, and 47.7 mm in SR₃₀, SR₄₅, SR₆₀, BMR₃₀, BMR₄₅, BMR₆₀, CMR₃₀, CMR_45, and CMR₆₀ (SR, BMR, and CMR were ridges with manually compacted soil, covered with bio-degradable mulch film, and covered with common plastic film, respectively, and subscripts 30, 45, and 60 refer to ridge widths (cm), all with 60 cm furrow width), respectively. There was no distinctive dry soil layer found within the root zone among ridge-furrow rainwater harvesting treatments during 2 consecutive years. Compared with TFP, forage yields decreased by 3%, 8%, and 13% for SR₃₀, SR_45, and SR₆₀, respectively, while water use efficiency (WUE) increased by 52%, 58%, and 55% over the two years in these ridge-furrow systems. Forage yields increased by 14%, 12%, 7%, 17%, 19%, and 9% for BMR₃₀, BMR₄₅, BMR₆₀, CMR₃₀, CMR₄₅, and CMR₆₀, respectively, while WUE increased by 49%, 62%, 59%, 51%, 67%, and 56% in these ridge-furrow systems in the same period. Optimum furrow width was 35-36 cm for BMR and CMR in the ridge-furrow rainwater harvesting system with 60 cm furrows for alfalfa production in this region that has an annual 380.7-427.6 mm rainfall. This information is useful for alfalfa production in ridge-furrow rainwater harvesting systems in this region.