The invention provides flue gas desulfurization gypsum-based thermal insulation mortar and a lightweight cold-bending thin-wall type steel composite wall. The flue gas desulfurization gypsum-based lightweight cold-bending thin-wall type steel composite wall comprises channel steel arranged at the bottom, multiple uniformly distributed cold-bending thin-wall type steel columns are vertically solidified on the channel steel, wall panels are installed on the two sides of the columns, the flue gas desulfurization gypsum-based thermal insulation mortar is poured into a cavity between the wall panels, and the flue gas desulfurization gypsum-based lightweight cold-bending thin-wall type steel composite wall is formed after solidification is achieved. The flue gas desulfurization gypsum-based thermal insulation mortar is prepared from flue gas desulfurization gypsum, cement, fly ash, lightweight aggregates, a water reducing agent, a retarder, a thickening agent and water, wherein the lightweight aggregates are polyphenyl particles or expanded and vitrified small balls. By means of addition of the lightweight aggregates, the weight of the material itself can be decreased to a certain extent, and the strength can be improved; due to the fact that the lightweight aggregates exit, it can be guaranteed that the size of the thermal insulation mortar is stable, the phenomenon that the material is separated from the wall panels due to the fact that the material is dried and then shrinks is avoided, it is guaranteed that the wall is tightly attached to the wall panels, and the comprehensive load capacity of the wall is improved.