The invention relates to low-modulus high-damping rubber for a building. Raw materials comprise a crude rubber matrix, carbon black, a plasticizer, a flame retardant, a damping filling material, an anti-aging agent, an antioxidant and a cross-linking agent; the crude rubber matrix is halogenated butyl rubber, and is prepared from chlorinated butyl rubber and brominated butyl rubber; the damping filling material is graphene or a fatty-acid metal salt or is prepared from the graphene and the fatty-acid metal salt. The crude rubber matrix is first subjected to thin-passing for 5 to 10 times by adopting an open mixer or an internal mixer; the carbon black, the plasticizer, the flame retardant, the anti-aging agent, the antioxidant, the damping filling agent and the cross-linking agent are added into the crude rubber matrix in sequence according to a proportion, and an obtained mixture is uniformly mixed; when the antioxidant is added, a mixing temperature is controlled to be 100 to 120 DEG C, a mixing temperature at other time is controlled to be 70 DEG C or below, and an overall mixing time is controlled to be 10 to 20 minutes; afterwards, a proper amount of mixed rubber is put into a corresponding mold, and is subjected to vulcanization molding in a press vulcanizer. The low-modulus high-damping rubber has the hardness which is 35 to 40 (shore A) while having high damping, and is low in horizontal stiffness; the shock insulation effect cannot be decreased.