To fully understand the sensitivity of photosystem II (PSII) activity of immature and mature leaves in desert plants to light stress, we explored the chlorophyll a fluorescence kinetics in Alhagi sparsifolia Shap. Plants. Results show that the chlorophyll fluorescence transient intensity of both immature and mature leaves was significantly higher under shaded conditions (about 50 % of natural light intensity) than that under ambient light conditions (100 % of natural light intensity). The energy fluxes per reaction center and cross section of immature and mature leaves were higher under shaded conditions than of those under ambient conditions. By contrast, the quantum yields and efficiencies and performance indices under shaded conditions were lower than of those under ambient conditions. However, the chlorophyll fluorescence transient intensity and parameters of both immature and mature leaves significantly changed under natural light exposure. These results suggest that the PSII activity of immature and mature leaves under shaded irradiance was enhanced, but the PSI acceptor side was affected as compared with that under ambient conditions. Activity of the donor side of PSII of immature leaves was higher than that of mature leaves in both shaded and ambient light conditions, and the electron transport activity at the acceptor side of PSII of both immature and mature leaves was inhibited by high irradiance after being kept in the shade. Furthermore, immature leaves suffered from severe inhibition than did mature leaves after exposure to ambient conditions. This study showed that immature leaves were more sensitive than mature leaves were to light stress.