The diploid black locust (diploid Robinia pseudoacacia) is a pioneer species of China's soil and water conservation forest, has strong adaptability and resistance and plays an important role in improving the ecological environment, preventing soil erosion and regulating hydrological conditions. The tetraploid black locust, also known as the polyploid black locust, is a double species of the diploid black locust. It was introduced from South Korea, with fast-growing, saline-alkali tolerant, drought-tolerant, dust-resistant and other characteristics. At present, studies on the tetraploid black locust focus mainly on cultivation and breeding techniques, however, the relationship between the ultrastructure of its leaves and its salt tolerance is still a lack of coverage. This study compared the changes in the leaf chloroplast ultrastructures of the diploid and tetraploid black locust under NaCl and Na₂SO₄ stress, which can not only identify their salt tolerance but also explore the function mechnism of salt stress to different plants.The ultramicrostructure of the chloroplast in the tetraploiddiploid black locust was observed under NaCl and Na₂SO₄ stress during 20 d. Before NaCl treatment, its chloroplast appeared spindle in shape, full in form and complete in structure. After NaCl treatment for 10 d, the chloroplast of the diploid black locust showed distortion, ambiguity in membrane, loose in grana lamella, disintegration in thylakoid, increases in lipid ball and so on. After NaCl stress for 20 d, the chloroplast of the diploid black locust displayed swelling and distortion, fracture in grana lamella and disintegration in membrane system. After Na₂SO₄ stress for 10 d, the chloroplast of the diploid black locust exhibited swelling, ambiguity in membrane, loose in grana lamella, disintegration in thylakoid. After Na₂SO₄ stress for 20 d, all membrane systems of the chloroplast in the diploid black locust were disintegrated and its chloroplast structures were damaged more seriously. On the whole, the ultramicrostructure of chloroplast of tetraploid black locust after salt stress had no obvious changes, only after Na₂SO₄ stress for 20 d, its chloroplast appeare hollow, loose in grana lamella and ambiguity in membrane edge.Before salt stress, the chloroplasts in two kinds of black locust were close to the cell wall and were distributed at the edge of the cell wall. After NaCl stress for 10 d, The chloroplasts of the diploid black locust were still in orderly rows and close to the cell wall, however, after NaCl stress for 20 d, most of the chloroplasts were out of the cell wall and were randomly distributed. After Na₂SO₄ stress for 10 d, part of the chloroplasts in the diploid black locust were breaked away from the cell wall and were located in the middle of it. After Na₂SO₄ stress for 20 d, most of the chloroplasts were out of the cell wall. Before and after two kinds of salt treatment, the chloroplasts in tetraploid black locust had no obvious changes in arrangement, were distributed at the edge of the cell wall and were close to it. Therefore, under salt stress, the chloroplast ultrastructure of salt-tolerant plants appeared intact, clear in grana lamellae and complete in thylakoid. The chloroplast ultrastructure of salt-intolerant plants displayed loose, deformation, fuzzy in grana lamellae, when these plants were severely damaged, their grana lamellaes were distorted, their chloroplasts were disintegrated and out of intact structures.