[Objective] Chlorophyll fluorescence characteristics of Bothriochloa ischaemum in response to soil water periodical change were studied in order to provide a theoretical basis for the rational utilization of forest and grass measures for soil and water conservation in loess hilly regions.[Methods] Seven mixture ratios (B. ischaemum to Lespedeza davurica, 0:12, 2:10, 4:8, 6:6, 8:4, 10:2, and 12:0) and three soil water regimes[HW 80%±5% of field capacity(FC), MW 60%±5% of FC, LW 40%±5% of FC)] were set in the pot experiment, and soil water content was reduced separately from HW to MW (H-MW) or from HW to LW (H-LW) during the heading period, flowering period, and mature period, respectively. Chlorophyll fluorescence parameters of B. ischaemum were measured at 0, 2, 4, 6, and 8 days after soil water reduced.[Results] Maximum photochemical efficiency (maximam fluorescence/variable fluorescence, F_v/F_m), active photochemical efficiency (Ф_PSⅡ) and apparent electron transport rate (ETR) showed decreasing trends with the water stress decrease, and those values decreased much more under H-LW-3. F_v/F_m and Ф_PSⅡ in the heading period or the flowering period were signicantly higher than in the mature period, and the F_v/F_m was larger than 0.750. Non-photochemical quenching (NPQ) showed increasing trends in the heading period and flowering period, and decreasing trends in the mature period, but the photochemical quenching (q_p) showed opposite trends during the growth period. After 8 days of soil water reduction, F_v/F_m and Ф_PSⅡ in the mature period under H-MW were signicantly higher than under H-LW, and F_v/F_m, Ф_PSⅡ, and ETR in the 12:0 mixture ratio were signicantly lower than in others under most water treatments.[Conclusion] The application of mixture planting enhanced the PSⅡ photochemical activity of B. ischaemum when intercropped with L. davurica under periodically decreasing soil water conditions.