Through field investigation and soil fertility index analysis,causes and mechanism of fertility degradation of black soil in the Songliao Plain were explored.The results show that the interface between the cultivated layer and the plough sole in the soil profile was undulated under the current cropping system mainly using small-sized four-wheeled tractors and flat under the cropping system of deep plowing annually in the fall.Besides,the two systems were significantly different in soil nutrient content and soil physical and chemical properties.The study also discovered that both the present fertilization system based on inorganic N and inorganic P and the present system of continuous cropping of corn had greats effect on balance of soil nutrients. It thus follows that the present improper systems of cultivation,fertilization and continuous corn cultivation are the main causes of fertility degradation of black soil in the corn belt of the Songliao Plain.