Method of fluorescence induction kinetics was used to investigate the relationship between light energy utilization and dissipation of PSⅡ of Puccinellia tenuiflora seedlings and osmotic potential of culture solution under Na₂CO₃ stress. It was found that the changes of maximal photochemical efficiency of PSⅡ (Fv/Fm), potential photochemical efficiency (Fv/Fo), actual photochemical eff iciency of PSⅡ in the light (ΦPSⅡ) and photochemical efficiency of PSⅡ in the light (Fv′/Fm′) were not obvious in an osmotic potential higher than -4 bar. At the same time, Fv/Fm, Fv/Fo, Fv′/Fm′ increased along with the osmotic potential lower than -4 bar; nevertheless ΦPSⅡ, electron transfer rate (ETR), photochemical efficiency, heat dissipation and heat dissipation rate decreased. The results showed that the dissipation mechanism of surplus of light energy of P. tenuiflora seedlings may be various in different osmotic potential induced by Na2CO3 stress (lower than -4 bar and higher than -4 bar), and there may be a refined osmotic regulation mechanism under osmotic potential higher than -4 bar. Furthermore, in high osmotic stress induced by Na2CO3, the safeguard mechanism of P. tenuiflora was different from that of other plants, they dissipated surplus light energy via two approach: increasing light energy absorbed by light-harvesting pigment that used in heat dissipation and heat dissipation rate; increasing ΦPSⅡ, ETR and photochemical efficiency, and building up fake circulate photosynthetic phosphorylation, which resulted in the augment of active oxygen eliminated by antioxidant enzymes system. In this way do P. tenuiflora protect photosynthetic apparatus from the damage of surplus of light energy.