The invention discloses a porous manganese slag-vitrified microsphere composite inorganic thermal-insulation mortar. The thermal-insulation mortar employs lightweight thermal-insulation aggregate (a vitrified microsphere and porous manganese slag), cementing materials and modification additives as main raw materials, wherein the vitrified microsphere and the porous manganese slag are used as a thermal-insulation main body, cement, micro steel slag powder, fly ash and limestone are used as the cementing materials, and dispersible latex powder, hydroxypropyl methyl cellulose, a wood fiber, a polypropylene fiber and a sisal fiber are used as the modification additives; the above-mentioned materials are mixed to form a pre-mixed dry powder thermal-insulation mortar. The raw materials of the thermal-insulation mortar comprise, by mass, 35 to 65% of the inorganic cementing materials, 15 to 45% of the vitrified microsphere, 20 to 45% of a manganese slag particle and the additives with total mass accounting for 2.9 to 4.8% of the mass of the inorganic cementing materials. The inorganic thermal-insulation mortar prepared in the invention has the characteristics of high strength, a small heat conduction coefficient, a low dry shrinkage rate, fire resistance, resistance to biological bite and corrosion, greenness, environmental protection, etc. and is applicable to thermal insulation of exterior walls of a variety of buildings, thermal insulation and heat preservation of roofs and flooring and other auxiliary thermal insulation.