The invention discloses phosphorus building gypsum base light self-leveling mortar. The phosphorus building gypsum base light self-leveling mortar is characterized by being prepared from the followingcomponents in percentage by mass: 70 to 90 percent of phosphorus building gypsum, 0 to 20 percent of dried sand, 0 to 20 percent of cement, 0 to 5 percent of coal ash, 5 to 20 percent of lightweightaggregate, 0 to 15 percent of active excitant, 0 to 3 percent of water repellent, 0.1 to 1 percent of high-performance plasticizer, 0 to 0.2 percent of sedimentation-resistant assistant, 0 to 0.2 percent of flocculation assistant, 0.01 to 0.4 percent of calcium sulphate retarder, 0 to 3 percent of high-molecular polymer and 0.05 to 0.3 percent of high-performance de-foaming agent. The prepared light self-leveling mortar is less than or equal to 1,000Kg/m<3> in unit weight, is larger than or equal to 10MPa in pressure resistance. The production process is easy, is low in energy consumption, andis high production performance; the light self-leveling mortar is particularly suitable for indoor thick layer leveling projects, has certain insulation, heat insulation and sound insulation functions, contributes to lowering environmental pollution, is simple in production process, is low in energy consumption and has high performance; compared with other self-leveling mortar products, the phosphorus building gypsum base light self-leveling mortar has higher market competitivity and high practicability.