Abstract: Hydroponic experiment was conducted to investigate the effect of sodium nitroprusside (SNP, an exogenous nitric oxide donor) on chlorophyll fluorescence characteristics in tomato seedlings under 100 mmol·L^-1 NaCl stress. The results show that SNP increase maximum fluorescence (Fm), photochemical efficiency of photosystem II (Fv/Fm), potential photochemical efficiency (Fv/Fo), electron transport rate (ETR), actual photochemical efficiency (ΦPSⅡ), photochemical quenching coefficient (qP) and photochemical efficiency (Prate). However, SNP decreases minimal fluorescence (Fo), non-photochemical quenching coefficient (qN), antenna heat dissipation rate (Drate), excitation pressure (1-qP) and the deviation from full balance between PSⅠand PSⅡ(β/α-1) in tomato seedlings under NaCl stress. This then suggests that nitric oxide reduces excess excitation dissipation, improves photochemical electron transport efficiency and efficiently protects tomato leaf from PSⅡdamage under salt stress.