In order to investigate the prolonged actions of biochar on soil fertility and soybean yield in sloping farm land of the black soil region of Northeast China, an experiment about the prolonged actions of biochar in sloping farm land with three typical slope gradients was conducted in 2016—2018. The prolonged affects of biochar on soil agglomeration and its stability, soil nutrients, soybean yield and its components were analyzed, and the soybean yield was predicted based on an improved grey theory prediction model. And then the affecting period of biochar on yield increasing after onetime application was clarified. The results showed that the contents of soil aggregates with diameter of d<0.25mm and d>0.25mmwere significantly decreased and increased, respectively. The application of biochar made the waterstable aggregate content ratio R0.25, the average mass diameter (MWD) and the geometric mean diameter (GMD) all increased and the soil unstable group LT grain index (ELT) decreased, that was the stability of soil aggregates was decreased with the increase of land slope gradients and prolonged period after biochar application. Biochar made the pH value, ammonium nitrogen, available potassium and organic matter content increased significantly(P<0.05), the highest increasing rate of them were 17.88%, 27.23%, 20.31% and 17.51%, and the soil nutrient level was risen, and soil fertility was improved after biochar application, and the effect of them was negatively correlated with the years of biochar application. However, biochar had no significant effect on available pH contents. After applying biochar, the number of pods and grains for per plant, the quality of 100 grains, and yield were significantly increased (P<0.05), and the yield was increased by 26.29%. The bigger the land slop gradients and the longer the biochar applying were, the smaller the increasing amplitude of each index was. The order of the influence degree of various factors on soybean yield from big to small was biochar application, slope gradients and years after biochar application. The improved multivariate grey model had higher precision of yield prediction, and the effective time of biochar for soybean yield increase should be 5~6 years after biochar application. The results could provide a theoretical basis for the application of biochar in the black soil region of Northeast China.