To clarify the physiological characteristics of switchgrass as an introduced species in semiarid region, we investigated its leaf fluorescence characteristics when mixed with old world bluestem at five densities (i.e. 0∶8, 2∶6, 4∶4, 6∶2,8∶0) under shortterm water stress ［soil moisture contents declined from 80% to 20% FC (field capacity is 80%) and then rehydrate to 80% FC］and two nitrogen treatments (N₀ and 0.1g N·kg^-1 addition) in a pot experiment. Results showed that: (1) The maximal photochemical efficiency (F_v/F_m), PSⅡ actual quantum yield (Φ_PSⅡ), apparent electron transport rate (ETR) and photochemical quenching coefficient (qP) of switchgrass decreased gradually with soil water drying, and recovered to the same level as with the control plants in the second day after rewatering. (2) ETR value of switchgrass was significantly higher in monoculture under both N fertilizer treatments, and qP value was significantly higher in monoculture than those in mixture under nitrogen addition treatment, and vise versa for nonphotochemical quenching coefficient（NPQ) value, indicating that the activity of PSⅡ reaction declined in mixture, and switchgrass had higher competition ability under mixture. (3) Φ_PSⅡ and qP values of switchgrass increased 13.64%-23.53% and 6.12%-11.11%, respectively, and NPQ value decreased 9.76%-12.82% under N addition treatment, which indicated that N could improve the light energy use efficiency and competitive ability of switchgrass. Our results suggested that switchgrass have strong adaptation when mixed with old world bluestem under different soil water and N application treatments, and N addition could increase its competitive ability to the component species.