The invention discloses a soil amendment for a cold waterlogged paddy field, which comprises two components of A and B; the A component is mixed by the following raw materials by weight: 50-60 parts of stove ash, 10-15 parts of blast furnace slag, 20-25 parts of attapulgite, 3-5 parts of gypsum, 3-5 parts of calcium magnesium phosphate fertilizer, 0.2-0.5 parts of zinc sulfate, and 0.2-0.5 parts of borax; the B component is biomass char which is obtained by anaerobic carbonization of rice husk, corn straw, and the like at 300-450 DEG C. The soil amendment of the invention can increase the temperatures of soil and water, improve soil permeability, reduce reductive substance contents, and realize yield increase of more than 20% for paddy rice yield. The original creation of the invention is that after rice transplanting, the biomass char is directly broadcasted on the surface of field water; with the characteristics of low density and deep colors for biomass char, the biomass char is floated on the water surface to absorb sun energy, and thus to increase the temperatures of soil and water, and to improve the cold problem for cold waterlogged paddy fields. The biomass char is rich in nutrients such as phosphorus, potassium, and the like; after the biomass char is broadcasted on water surfaces, nutrients such as phosphorus, potassium, and the like can dissolve in water, and then directly be absorbed and used by crops, which reduces soil adsorption and fixation, increases nutrient utilization efficiency, and promote paddy rice early-stage growth.