The invention discloses a method for biologically improving heavy saline-alkali soil and achieving high-yield and high-quality planting by adopting barnyard grass. The method is implemented in two years and comprises the following steps: 1) selecting an appropriate barnyard grass variety for later use in the first year; 2) carrying out deep ploughing, winter irrigation, fertilizer salt application and shallow ploughing: deeply ploughing the soil by more than 25cm before late September, ensuring the irrigation amount to be 700-800m<3>/667m<2> in early October each year, spreading 50-60kg of phosphate fertilizer (the P2O5 content is 12%) on the surface of the soil in the middle of October and shallowly ploughing the soil by 10-12cm; 3) carrying out soil preparation, ridging and film covering: in the next year, according to the climatic conditions, after soil rotary tillage, preparation and compaction in late March or in early April, covering the soil with 1.45m black plastic films, wherein the film surface is 1.2m and the distance between two films is 0.4m; 4) in the next year, not deeply ploughing the soil, retaining the mulching films, implementing no-tillage measures and ensuring the irrigation amount to be 700-800m<3>/667m<2> in early October; 5) carrying out the steps of variety selection, seeding, topdressing and irrigation and timely harvest in the same way as in the first year. The method has the beneficial effects that the barnyard grass is wide in distribution areas, can be applied in heavy saline-alkali areas in China to improve the local saline-alkali soil and an extensive popularization prospect.