The invention discloses a magnesium oxysulfate cementitious material for 3D printing houses. The cementitious material is mainly prepared from light calcined magnesia, magnesium sulfate heptahydrate, fly ash, phosphorus slag, pitch-based carbon fibers, medium-alkali glass fibers, anhydrite, water glass, calcium carbonate, a water reducing agent, a coagulation accelerator, a retarder, metakaolin, silica fume, diatomite, sodium formate, mirabilite and a modifier. According to the magnesium oxysulfate cementitious material for the 3D printing houses, the formula is reasonable, the materials are convenient to obtain, the adhesive property is good, the stability is high, the good pump-out morphology keeping capacity and adhesive property are achieved, the printed buildings have the excellent properties such as light weight, high strength, fire resistance, flame retardance, water resistance, immersion resistance, sound absorption, sound insulation, heat preservation, heat insulation, good size stability and upper high weather resistance, the requirements of the 3D printing buildings on construction continuity and building strength are meet, and the house buildings achieve the good overall stability and using safety. In addition, a preparation method is simple and convenient to implement.