Using variety ‘Yanke No. 2’ as the test material, we used the pot water to set normal water supply (75% field water holding capacity), moderate drought stress (60% field water holding capacity and severe drought stress (45% field water holding capacity). Chlorophyll fluorescence technology was used to study the growth of oats and the changes of leaf photoreaction system Ⅱ (PSⅡ) function under different water gradients, and to explore the effect of drought stress on the photosynthetic performance of oat leaves. The results showed that: (1) drought stress caused the height of the oat plant to become shorter, the number of leaves, main stems, and ears decreased, the leaves became green and yellow, and the grain yield decreased significantly. (2) Compared with normal water supply, the maximum photochemical efficiency (F_v/F_m) and photosynthetic performance index (PI_ABS) of oat leaves were significantly reduced under severe drought stress. (3) Severe drought stress caused the light energy absorbed by the unit reaction center (ABS/RC) and the energy dissipated by the unit reaction center (DI₀/RC) of the oat leaves to decrease significantly. The energy used by the unit reaction center for electron transfer (ET₀/RC) and the light energy captured by the unit reaction center (TR₀/RC) increased significantly; the openness of the active reaction center (Ψ₀) and the quantum yield of the electron transfer chain (φ_E0) decreased significantly, and the maximum quantum yield was nonphotochemically quenched The forehead (φ_D0) is significantly increased, the relative fluorescence intensity at the three sites of V_J, V_K, and V_L is significantly increased, and the initial slope M_o of the OJIP curve is significantly increased. The study found that the PSⅡ of oat leaves has a strong adaptability to moderate drought stress, and severe drought stress severely damages the PSⅡ reaction center of its leaves, resulting in an imbalance in the energy flow distribution of its reaction center, blocked electron transfer and weakened PSⅡ stability, which in turn affects the photosynthesis of oats eventually leads to the inhibition of oat growth.