The invention discloses a method for preparing a compactly coated silicon-carbon nanocomposite and belongs to the technical field of lithium battery cathode materials. The method mainly comprises thesteps that a thermal plasma method is utilized to prepare spherical nano-silicon; and according to hardness characteristics of soft and hard carbon and silicon, a ball-milling method is utilized to perform ball-milling on the nano-silicon prepared through the plasma method and soft and hard carbon materials to obtain a silicon-carbon cathode material with high carbon coating performance. Accordingto the method, the hardness characteristics of the materials are introduced, and the compact lithium battery cathode silicon-carbon composite is prepared according to the characteristic that the materials different in hardness are different in acting force in the ball-milling process. Through the composite, the influence of expansion of a huge volume of silicon is obviously relieved, a stable SEIfilm is formed, conveying channels of ions and electrons are broadened, and the composite has better electrochemical properties.