Abstract: To explore the physiological response mechanism of plant photosynthesis under different afforestation modes and improve the plantation forests precisely.In this study, Li−6400XT was used to determine the diurnal changes and light response curves of leaf photosynthesis of Pinus tabuliformis, Forsythia suspensa, and Caragana korshinskii in pure and mixed forests in loess hilly areas. The results showed that the afforestation model did not affect the daily change trend of the net photosynthetic rate(P_n) of the 3 plants and tree species, but only affected the average daily P_n change of the 3 plants species. Pine(mixed Forsythia suspensa) > Pine(mixed Caragana korshinskii) > Pine(pure forest), Forsythia(pure forest) > Forsythia(mixed Pine), Caragana korshinskii(mixed Pine) > Caragana korshinskii(pure forest); in different afforestation modes, only the relationship between P_n and air relative humidity shows a difference; when the photosynthetically active radiation(PAR) is 2000 μmol/(m²·s), P_n of all tested plant leaves tends to be saturated. Compared with pure forest, the P_n of Pine(mixed Forsythia suspensa),Pine(mixed Caragana korshinskii) and Caragana korshinskii(mixed Pine) increased by 42.1%, 10.5% and 74.0%, respectively, and the P_n of Forsythia(mixed pine) decreased by 40.4%; four light response models are used to fit the light response curve, and the results show that the right-angle hyperbolic correction model(MRH) has the best effect after fitting. The photosynthetic parameters of Pine(mixed Forsythia suspensa) and Pine(mixed Caragana korshinskii) are higher than Pine(pure forest), and Caragana korshinskii(mixed Pine) is higher than Caragana korshinskii(pure forest). Mixed afforestation is more beneficial to the photosynthesis of Pinus tabulaeformis and Caragana korshinskii and can increase the plant environment adaptability and promote plant growth.