The invention relates to a method for directly producing low-molecular weight heparin sodium through a heparin sodium crude product, and the method comprises the following steps of: (1) dissolving the heparin sodium crude product into a heparin sodium crude product solution by using a sodium chloride solution, regulating the pH value of the heparin sodium crude product solution to 5.0-10.0 by using hydrochloric acid or a sodium hydroxide solution, centrifugalizing by using a high-speed centrifuge to remove undissolved substances, depositing liquid obtained through centrifugalization by using ethanol, standing the deposited liquid at low temperature, collecting a sediment A, diluting the sediment A by using purified water, adding hydrogen peroxide for oxidation after the sediment A is completely dissolved, filtering, adding the ethanol for deposition, and standing to obtain a sediment B; and (2) taking out the sediment B as the intermediate of the heparin sodium crude product, adding the purified water for dissolution, stirring, adding sodium nitrite to degrade heparin sodium at the same time, adding sodium borohydride to reduce the heparin sodium, intercepting large molecules by using an ultrafilter after the reduction is finished, adding the ethanol to ultrafiltrate for deposition, dewatering by using the ethanol, and drying to obtain the low-molecular weight heparin sodium.