As the second most abundant mineral element in the soil, silicon has been found to alleviate the adverse impact of drought in many plant species and increase the water use efficiency in others. If the water use efficiency of alfalfa could be enhanced by simply adding silicate to the soil, this measure could potentially be applied in pasture management to face rising irrigation costs and water demand from inadequate supplies of surface water, together with declining water tables. Therefore, the effects of addition of silicon on the water use efficiency and forage yield component of alfalfa (Medicago sativa L.) were investigated across four soil moisture environments. This study shows that addition of silicon improved water use efficiency of alfalfa under lightly (65% of field water capacity) and moderately (50% of field water capacity) water-stressed conditions, by decreasing the transpiration rate （p<0.05） but it had no effect on alfalfa water use efficiency under wet (80% of field water capacity) and seriously(35% of field water capacity) water-stressed conditions. This study also indicated that addition of silicon significantly increased the forage biomass by promoting shoot development (p<0.05) and enhancing the plant height under the moderately water-stressed condition and only by enhancing the shoot development under the slightly water-stress rather than biomass per branch (p<0.05). The results of this study suggest that improvement of water use efficiency of alfalfa by added silicon is regulated by soil moisture.