The invention relates to a curing agent and a construction method of weak soil. The curing agent of the weak soil is characterized by comprising powder and liquid, wherein the powder comprises cementand congealing stones or the following materials in parts by weight: 0-20 parts of cement clinker, 60-95 parts of grain slag or slag, 1-5 parts of lime, 1-5 parts of plaster, 1-5 parts of anhydrous sodium sulphate, 0-10 parts of caustic soda, 0-10 parts of water glass and 0.1-3 parts of sodium fluosilicate; the liquid comprises the following components in parts by weight: 1-15 parts of polyacrylamide, 20-60 parts of pulp waste liquid, 10-40 parts of cement retarder, 0.5-1.5 parts of OP emulsifier, 0.3-1.0 part of carrageenan, 0.3-1.0 part of xanthan gum and 0.5-1.5 parts of alpha-olefin sulfonate. The curing agent is previously prepared into seriflux before being used, and the weak soil and the curing agent seriflux are uniformly stirred by mechanical stirring equipment and then are spreador compacted. The addition quantity of the powder is greatly reduced because a small quantity of the liquid is added; in addition, materials can be easily obtained, and the performance of consolidated soil is superior, thus the curing agent of the weak soil, i.e. sludge and the like, has high efficiency and environmental protection.