The invention relates to a method for quickly fertilizing and improving barren farmland soil. The biogas slurry waste residue, humic acid, straw mulching and water-retaining agent are mixed to form a fertilizing material and then the fertilizing material is applied to the farmland in the form of base fertilizer. According to the invention, the organic matter, nitrogen, phosphorus and potassium contents of the soil can be obviously increased; the soil can be quickly fertilized; the crop yield and the water use efficiency can be obviously increased; the method can quickly become effective and is easy to popularize.