The invention relates to the fields of ecological environment protection and chemical production, and relates to a water-retaining agent used in saline lands, and a preparation method thereof. The water-retaining agent is composed of substances of, by weight: modified corn bran, industrial-grade acrylic acid, industrial-grade acrylamide, a neutralizing agent, a water-soluble catalyst, industrial-grade purified water, a cross-linking agent, a stabilizing agent, ultrafine modified bentonite powder with a specification of 1200-1800 meshes, and sorbitol tristearin. The invention also discloses a preparation method of the water-retaining agent. The water-retaining agent provided by the invention is advantaged in high reaction efficiency, low cost, and good application effect in saline land ecological vegetation restoration.