The invention belongs to the technical field of building materials, and particularly relates to high-viscosity water-retaining plastering mortar taking a phosphogypsum hydraulic composite cementing material as a main material and a preparation method thereof. The plastering mortar is prepared from ordinary silicate cement, sulfur-aluminum cement, phosphogypsum, bauxite, red mud, quartz sand of 30 to 50 meshes, quartz sand of 40 to 70 meshes, a polycarboxylic acid type water reducer, sodium borate, re-dispersible latex powder, a water retaining agent and an anti-foaming agent. A product prepared by the invention is suitable for ground plastering mortar which has high requirement on fluidity, and has high fluidity, high viscosity and high water retaining property. By adopting the preparation method, a large amount of phosphogypsum is consumed, so that the problem that the ecological environment is contaminated due to piling of the industrial waste of phosphogypsum is solved; meanwhile, the production cost of the plastering mortar is lowered by adopting the phosphogypsum instead of natural gypsum, so that the plastering mortar has high economical benefit, social benefit and environmental protection benefit.