The understanding on the seasonal variation of light use efficiency (LUE) in desert shrubs and its regulatory factors, especially its biological ones, is very limited, leading to uncertainty in the productivity model of desert ecosystems. We hypothesize that the seasonal change of daily mean LUE (LUE_day) of Artemisia ordosica is related to the seasonal adjustment of leaf traits in a long-term drought environment. To test this hypothesis, LUE_day and key leaf functional traits of Artemisia ordosica were measured with a portable photosynthetic system and their relationships were analyzed during the growing season. The seasonal fluctuation range of LUE_day was 0.003-0.017 mol/mol, and the coefficient of variation (CV) of LUE_day was 38.75%. The mean of LUE_day at leaf expanded period was 17.37% lower than that at the whole growing season, and about 30% lower than that at leaf expanding period and leaf defoliation period. Eight leaf traits showed different seasonal variation degrees, the CV values of total chlorophyll content (Chl), Carotenoid content (Car) and leaf nitrogen content (LNC) were generally higher over 20%, and the CV values of leaf carbon content (LCC) and leaf relative water content (LRWC) were less than 7%. LRWC was significantly correlated with all leaf biochemical traits (i.e., Chl, Chl a/b, Car, LNC and LCC), indicating that the changes in LRWC were tightly related to the transport process of nutrient absorption, photosynthetic pigment synthesis and carbon assimilation in leaves. The relative changes of LUE_day of Artemisia ordosica were significantly positively correlated with LRWC, Chl a/b and LNC. The seasonal variations of LRWC and LNC were mainly regulated by air temperature (T_a) and soil volume water content (VWC). Moreover, the changes of Chl a/b were mainly controlled by 10 cm VWC. Considering many studies predict that more extreme climate events (e.g., extreme drought and continuous heat wave events) will occur in the dryland in the future, these findings emphasized that the seasonal adjustment of plant leaf traits response to drought and high temperature should be fully considered in the flux modeling scheme of dryland ecosystem. Our results will help construct a leaf scale photosynthetic model and understand the biological regulation mechanism of LUE in xerophyte species.