The invention discloses thermal insulation mortar which comprises the following components in percentage by weight: 20 to 60 percent of caustic calcined magnesia, 1 to 25 percent of fly ash, 5 to 35 percent of thickening time control agent, 0 to 40 percent of enhanced thickener, 1 to 20 percent of kieselguhr, 5 to 35 percent of expanded aggregate, 0 to 5 percent of polystyrene foam particles, 0 to 0.2 percent of polypropylene fiber, 0.5 to 8 percent of water-resisting admixture, and 0 to 2 percent of water reducing agent. The thermal insulation mortar has the characteristics of capability of coagulating and hardening quickly and having strong binding capacity with base materials, good volume stability, capability of producing volume micro-expansion in the process of coagulating and hardening, no crack, no splitting, strong water resistance, ageing resistance, long service life, light weight, and good heat preservation and heat insulation performance, wherein the coefficient of thermal conductivity is 0.05 to 0.30 W/m.k. The thermal insulation mortar has the advantages of no toxicity, odourlessness, zero radioactivity, no harm to human bodies, no pollution to environments, energy conservation, environmental friendliness and low production cost.