The invention discloses a special decorative mortar for lightweight thermal insulation walls, wherein the special decorative mortar comprises raw materials and water according to a mass ratio of 4.2-6.8:1-1.2, and the raw materials comprise, by weight, 30-50 parts of cement, 8-15 parts of calcined gypsum, 15-25 parts of slag micropowder, 20-30 parts of a polyphenyl particle and vitrified microbeadcomposition, 1-2 parts of polyurethane fiber, 3-8 parts of a polyacrylate emulsion, 0.3-0.8 part of a mortar water retention agent, and 2-5 parts of an additive. According to the present invention, the special decorative mortar has advantages of excellent adhesion, excellent water retention property, low thermal conductivity and excellent thermal insulation performance, can be used as the lightweight thermal insulation wall decorative mortar, and can well solve the problems of difficult lightweight thermal insulation wall plastering, easy cracking, bubbling, powder falling and shedding.