The effect of sodium hydrogen phosphate/citric acid mixtures on enhancing phytoremediation of heavy metal-contaminated soil by alfalfa (Medicago sativa L.) and the changes of metal availability in soils were investigated. Alfalfa plants grew healthily in heavy metals-contaminated soils (collected from molybdenum mine) for 30 days. The heavy metal content was determined by ICP-OES. In this paper, it was found that: the phytoextraction efficiency of alfalfa plants was enhanced after the sodium hydrogen phosphate/citric acid mixtures were added into soil. As usual, the phosphorus can promote plants to accumulate more biomass (increasing 0.35%-24.62%) even if the availability of metals to extraction were increased by citric acid in the soil. With the treatment of sodium hydrogen phosphate/citric acid mixtures, the bioconcentration factor (BCF) values of alfalfa plants and the acid soluble fractions of heavy metals in soils were increased. The translocation factor (TF) values of all alfalfas in heavy metals were low, but the TF of As, Cr, Hg, Mo tended to increase with sodium hydrogen phosphate/citric acid mixtures. In other words, both the availability and the biological-validity of heavy metals in soils were increased after sodium hydrogen phosphate/citric acid mixtures were added into soils.