Water potential is one of the most important factors that reflect environmental stress on plants, and can be used to identify the level of stress a plant experiences and its adaptation ability. This study focused on the coppice dunes formed by the psammophyte Spinifex littoreus on Pingtan Island, Fujian province. The PSYPRO Water Potential System was used to determine the water potential of the leaves, stems, and soil, and a HUMIPORT10 hand-held hygrometer was used to simultaneously measure the meteorological variables on cloudless, sunny days. Spinifex littoreus showed an opposite pattern of leaf water potential, which is in contrast to the normal phenomenon of increased leaf water potential in the morning and evening compared to that at midday. The diurnal variations of leaf and stem water potential all demonstrated an 'M’ curve pattern, indicating a decrease of leaf water potential and increase of stem water potential in daytime, until 18:00. Furthermore, we found that the changes of leaf water potential occurred later than those of stem water potential. The biggest discrepancy between stem and leaf water potential, reaching 0.65MPa, appeared at 10:00, when stem water potential was higher than leaf water potential. At about 14:00, an abnormal phenomenon appeared, in which leaf water potential was higher than stem water potential, but at about 18:00, leaf and stem water potential tended to be the same. With the exception of soil water potential at 30cm, the diurnal variation of soil water potential showed a decrease starting in the morning, which reached the minimum at 14:00; however, this tendency was not statistically significant(P > 0.1). With increasing depth, the soil water potential showed a tendency to increase, but this trend disappeared below a depth of 80cm. Strong correlations and significant changes (P < 0.1) in water potential were observed between the atmosphere water potential and leaf. Leaf, stem, and 30-cm-deep soil water potential showed some correlations, but the variations were not statistically significant (P > 0.1). Leaf water potential was not correlated with the water potential of soil at other depths.