The invention discloses a method for preparing composite absorbent by utilizing low-grade attapulgite clay. The method comprises the steps of uniformly mixing attapulgite clay mineral powder containing quartz and dolomite with soluble silicate accounting for 20 to 600 percent of the weight of the attapulgite clay mineral powder and metal salt accounting for 10 to 500 percent of the weight of the attapulgite clay mineral powder or a solution containing the metal salt, then grinding the mixture, dispersing the mixture into water in a solid-liquid ratio of 1:100-1:10, adding an appropriate amount of additive, and aging for 1 to 4 hours; then transferring the mixture into a hydrothermal reaction kettle to react for 5 to 60 hours at the temperature of 120 to 300 DEG C, and then separating, washing, drying and crushing to obtain the adsorbent. According to the method, the quartz impurities and dolomite in the attapulgite clay mineral powder are converted into amorphous silicate with strong adsorption activity, and the prepared adsorbent is large in specific surface area and uniform in aperture distribution, is used for adsorbing and removing dye, heavy metal, organic molecules, radioactive elements, rare earth and mycotoxin and is wide in application prospect in the fields of biology, food, petrochemical industry, environment restoration, printing and dyeing waste water treatment and the like.