2-year-old potted Starkrimson/M. hupenensis Rehd. apple trees were used as materials in the experiment Effect of potassium on water use efficiency and relevant parameters of plants under conditions of different soil water status was preliminarily studied. The results showed that under the condition of adequate soil water content, stomatal conductance (Gs),photosynthetic rate (Pn) and transpiration rate (Tr) were all improved for trees applied potassium fertilizer, which resulted in the reduction of WUE because Tr was increased more than that Pn was. However, under the condition of soil drought, WUE of trees applied potassium fertilizer was apparently promoted as compared with contrast ones. WUE of trees with different potassium level was increased in the order: high K > medium K > low K For treated plants under soil drought, as a result Gs and Pn were enhanced but Tr lowered, the improvement of WUE was due to both the increase of photosynthetic rate and the decrease of transpiration rate.