A reasonable and efficient rotation sequences were established to solve the problem of replanting obstacles caused by continuous cropping of crops via determining the level of soil nutrient immobilization and residue under annually fallowing and seasonally planting different crops. The nutrients contents of 0-20 cm black flax soil under fallowing, and planting spring wheat, potato, Mongolian astragalus and Angelica sinensis were investigated. The field trial was followed by the single-factor randomized block design in Zhanpo village, Lianfeng town, Weiyuan, Gansu in 2018 and soils were sampled by "S" shape. The results showed that the contents of organic matter, total nitrogen, available phosphorus, available potassium, total anion and anion, Mg²⁺, SO₄^2- and Cl^- in 0-20 cm soil layer respectively were increased for annually fallowing and seasonally planting Mongolian astragalus. however, the contents of organic matter, total nitrogen, available phosphorus, available potassium, total amount of ions and ions, Mg²⁺, SO₄^2- and Cl^- in 0-20 cm soil layer were decreased for annually planting Angelica sinensis. The variance analysis showed that the contents of organic matter, total nitrogen, available phosphorus, available potassium, total anion, Mg²⁺, SO₄^2- and Cl^- in 0-20 cm soil layer were significantly higher for annually fallowing and seasonally planting Mongolian astragalus than those for seasonally planting Angelica sinensis. The principal component regression analysis showed that the soil fertility composite index scores respectively were up to 0.98 and 0.56 for annually fallowing and seasonally planting Mongolian astragalus, while the soil fertility composite index score was down to 0.07 for seasonally planting Angelica sinensis. This demonstrated that annually fallowing was a simple and effective method for combination of land utilization and maintenance in black flax soil, while seasonally planting Mongolian astragalus was an effective rotation sequence to improve soil fertility in black flax soil.