The invention provides a palygorskite-clay (attapulgite) composite flocculating agent. The flocculating agent is obtained by compounding sodium metasilicate in a water phase by taking palygorskite-clay powder as a major raw material, taking polyacrylate sodium powder as a dispersing agent and taking magnesia powder as a suspension aid. As proved by an experiment, the palygorskite-clay composite flocculating agent prepared in the invention has very high adsorption capability on lignosulfonate in the water phase, and can be used for treating waste water containing lignosulfonate. The lignosulfonate is an anionic surfactant, so that adsorption treatment is equivalent to surface organic modification of an inorganic matter, sludge which adsorbs lignosulfonate can be used for treating other organic waste water or be added into a high molecular material for serving as a reinforcing agent, the recycling problem of sludge in waste liquid treatment is solved, and recycling of sludge is realized. Moreover, the flocculating agent takes a national special resource, i.e., palygorskite clay as a major raw material, is cheap and readily available, and has low cost and practical application value.