The photosynthetic physiological characteristics of plants can reflect their response to environmental conditions. To understand the photosynthetic adaptability of the major tree species for afforestation in a hilly loess region, the light response curves of Hippophae rhamnoides, Pinus tabuliformis, and Robinia pseudoacacia in pure and mixed plantations under different water conditions were measured in Ansai National Ecological Experimental Station, and the data fitted to five models. The results indicated that (1) the P_n values of these three tree species during a wet period (September) were significantly higher than those during a dry period (July) (P < 0.05), and there was photo-inhibition of photosynthesis in the H. rhamnoides pure plantation, R. pseudoacacia pure plantation, and R. pseudoacacia mixed plantation in July; (2) The sequence of fitting effect of the five light response models was, in descending order, as follows:modified rectangular hyperbola, modified exponential, non-rectangular hyperbola, exponential, then rectangular hyperbola. (3) The light response parameters (apparent quantum yield[φ], light compensation point[LCP], light saturation point[LSP], dark respiration rate[R_d], and maximum net photosynthetic rate[P_nmax]) for the three tree species in pure and mixed plantations in July were lower than those in September, suggesting that the light-use capability was diminished, and photosynthetic capacity was inhibited under dry conditions. (4) Compared with the pure forest, the photosynthetic physiological parameters of H. rhamnoides in the mixed forest showed an increased (in general), indicating that mixed afforestation was helpful to improving its photosynthetic potential, light use capability, and the light ecological amplitude. The photosynthetic physiological parameters in P. tabuliformis were reduced (in general), suggesting that mixed afforestation might reduce its photosynthetic capacity. Meanwhile, there was no obvious trend in the photosynthetic physiological parameters in R. pseudoacacia:the values of φ and P_nmax of the leaves decreased, while R_d increased, and the light ecological amplitude was widest, indicating that mixed planting might enhance its adaptability to strong lighting conditions, but it would reduce its ability to utilize the lower levels of light and photosynthetic potential.