Ammopiptanthus mongolicus (Maxim.ex Kom.) Cheng f., an ancient relic of the Tertiary Period, is a rare and endangered species of Fabaceae in Mid-Asia desert and has been listed as one of primary protection plants by the Chinese government. Because of its high value of study and ecological usefulness in defending desert, this plant has been called 'live fossil'. Using the leaves of A. mongolicus derived from water-stressed and water-supplemented environments at Shapotou in the south-east edge of Tengger Desert, seasonal changes of caloric values and osmotica, including free amino acids, total soluble sugar (TSS), sucrose, glucose and inorganic cations have been studied in this paper. The results showed that the main substances involved in osmotic adjustment varied with growth periods and between samples. At the early period it was soluble sugar mainly, and at the late period, besides soluble sugar, potassium and free amino acid (especially proline) also functioned. The higher levels of osmotica content (especially concentrations of total soluble sugar, sucrose, proline and potassium) and caloric values have been found in water-stressed plants rather than in irrigated ones.