A gypsum light mortar is a heat-insulating material of the inner wall of the building, the gypsum powder is taken as the gelled material which comprises light weight aggregate, polymeric additive and retarder, the light weight aggregate comprises expanded perlite, the polymeric additive is the combination of cellulose ether, lignocellulose, starch ether and polyvinyl alcohol, wherein, the expanded perlite is a silicon-processing modified expanded perlite. The mortar of the invention has the following advantages: the heat conducting coefficient of the mortar of the invention is greatly reduced, and the mortar has excellent heat preserving and insulating property, greatly-increased strength index comparing with the conventional heat-insulating mortar, excellent compression strength, securing the safety using, light weight, effectively reducing the sole weight of the building and the burden of the base load, same construction technique with the conventional coating mortar, simple operation; stable physical property, no hollow injection or cracking after construction, low activity, broad application field, and excellent compatibility and sticking force to various wall bases, the product of the invention in nontoxic, smell-less, inorganic and non-combustible.