The invention discloses polyacrylamide-modified soil improvement agent straw biomass carbon which is made from the following raw materials by weight: 82-84 parts of wheat straws, 17-19 parts of water hyacinth, 3.3-3.6 parts of calcium chloride, 1.4-1.8 parts of zinc sulfate, 2.3-2.6 parts of magnesium potassium sulfate, 1.8-2.1 parts of polyacrylamide, 1.2-1.4 parts of salicylic acid, 0.9-1.3 parts of sulfur, 3-5 parts of limonene, 4-6 parts of starch, 6-8 parts of desulfurization gypsum, a proper amount of a 10wt.% sodium hydroxide solution, a proper amount of a mixed acid solution and a proper amount of deionized water, first, the wheat straws and the water hyacinth are soaked and treated in the sodium hydroxide solution, then soaked with a calcium chloride and zinc sulfate compound mixed activator, carbonized at high temperature, acid-pickled, then treated by other means, and finally compounded with the limonene, the polyacrylamide and other ingredients to obtain the straw biomass carbon, when the straw biomass carbon is used in a soil improvement agent, water storage and retaining are excellent, and soil water use efficiency can be effectively improved.