[Background] Soil moisture is the key factor in determining the forest vegetation distribution especially in the northwest semi-arid area of China. Slope is basic spatial unit of forest distribution, and uneven distribution of soil moisture on a slope affects greatly forest vegetation. This work aims to understand the differences of soil moisture at varied positions of a slope. [Methods] A shady-facing slope, locating in Pailugou watershed, Qilian Mountains, was selected as example slope in order to analyze how soil moisture changed along the slope. During the growing season of 2015, 15 sites on 3 lines from top to bottom of the example plot were set up, there were five sites on each line, the upper slope position, upper middle-slope position, middle slope position, lower middle-slope position and lower slope position, and soil water storage at these sites were monitored every 10 days. [Results] 1) The mean values of soil water storage for whole soil profile (0 -80 cm thickness) (MSW) were calculated in the whole growing season. It was shown that the MSW changed with a parabolic curve from the upper to the lower along the slope with peak values of 201 mm and 198 mm on middle slope position and lower middle- slope position, respectively. The soil water storage on upper middle-slope and lower slope position was only 177 mm and 175 mm, respectively, and that on the upper slope position, i. e. , the highest slope position, was 159 mm, the lowest value of soil water storage. For daily soil moisture, the trend was similar to this along the slope, though the values of soil water storage difference among slope positions varied day by day. 2) The difference of soil water storage was not too much in May and August but significant in June, July, and September. For example, the difference of soil water storage between their minimum and maximum was 27 mm in May, and 65 mm in September. 3) And this trend of soil water storage did not change with weather conditions. Under the weather conditions without rain, with light rain or strong rain, the soil water storage for soil layer of 0 -30 cm was the lowest on upper position and the highest on middle slope position. For soil layer of 30 -80 cm, the soil water storage was the highest on middle slope position and lower middle-slope position under any weathers. The soil water storage was the lowest on upper middle-slope position under weather conditions without rain or only with light rain. Meanwhile, the lowest one occurred on upper slope position even under weather conditions with strong rainfall. [Conclusions] This work demonstrated that the middle and lower middle-slope position would store more water due to water redistribution although there was the same precipitation on the slope, which is conducive to make site selection and forestation based on the soil moisture difference among slope positions.