Alfalfa (Medicago saliva L.) is an important forage legume in farming and animal husbandry systems in semiarid areas. However, its forage yields in dryland farming areas are usually very low due to water shortages and low soil phosphorus (P) availability. This study assessed the effects of plastic film mulch and P fertilization on alfalfa forage yield and shoot P uptake in response to improved soil moisture and soil P availability in a dryland farming system of northwest China. A six-year field experiment, which commenced in 2011, was conducted at Yuzhong County, Gansu Province, China in randomized blocks of split-plot design with two mulch treatments-plastic film mulch with ridge and furrow (M) and no plastic film mulch with flat planting (NM)-as the main plots, and four P levels-0, 9.73, 19.3, 28.9 kg P ha(-1)-as split plots. Alfalfa forage yield and shoot P uptake and concentration increased linearly with increasing P level in the NM treatment. In the M treatment, shoot P concentration also increased linearly with increasing P level, but shoot P uptake and forage yield were well-fitted by a quadratic model with P level as the independent variable and a lower physiological P use efficiency at high P level. Alfalfa forage yield and water use efficiency increased with plastic film mulch. Soil water content in the upper 1 m of soil increased with plastic film mulch and was higher in the 2014 growing season and in April 2015 due to more rainfall. However, at 1-5 m soil depth, the soil water content decreased with plastic film mulch and P fertilization but increased at high P levels with plastic film mulch. Soil pH decreased with plastic film mulch and P fertilization, with a stronger effect from the latter, which is beneficial for improving soil P availability. Soil available P, mostly derived from inorganic P, increased with P fertilization and was lower in M than NM treatments due to alfalfa uptake and competition with soil microorganisms. We concluded that plastic film mulch and P fertilization, of about 16.1 or 17.5 kg P ha(-1), was an appropriate combination to improve soil water condition and soil P availability for increased alfalfa forage yield or shoot P uptake in this semiarid region.