Salt stress affects the photosynthesis of poplar seedlings. The levels of chlorophyll in the leaves decreased significantly with higher salt concentrations. Gas exchange parameters like Pn, Tr, and Gs decreased, while Ls increased initially then declined. Stomatal limitation caused photosynthesis to decrease with lower salt concentrations, while non-stomatal limitation was the main cause with higher concentrations. Chlorophyll fluorescence parameters also decreased with increasing salt concentrations. The salt tolerance of the poplar cultivars studied was ranked as P.×xiaozhuanica being the most tolerant, followed by Populus simonii×P.euphratica×P.sp24, and then Populus simonii×P.euphratica×P.sp19.