Abstract: The effects of silicon on cell ultrastructure of poplar leaves were investigated under salt stress for 30 days. The results showed that in the salt treatments without adding silicon, the slices of basal granule distorted or even disintegrated, the quantity of osmiophilic granule increased, the membrane of chloroplast became disintegrated and some of them disappeared with increasing external NaCl concentration and prolonging treatment time. Under salt stress，the cell nucleus were also condensed, and the nuclear membrane could not be seen clearly. The phenomena mentioned above was more obvious in the treatment with 4.0g/L NaCl than that in the 2.0g/L and 3.0g/L salt treatment. However, in the salt treatments with the addition of silicon, the degeneration of membrane systems decreased, and the lamellae were lined regularly, while the chloroplast grana lamellae just began to degenerate and the number of osmiophilic granule were increased more or less，even if some cell nucleus were condensed. The study also indicated that effects on mitigating salt stress were more significant in the treatment with silicon addition of 0.5g/L than in the addition of 1.0g/L. Moreover, the mitigating effects were more obvious in the 2.0，3.0g/L salt treatments than that in the 4.0g/L treatment.