The invention belongs to the technical field of building waterproof materials, and particularly relates to a viscosity-reducing flame-retardant type non-cured rubber asphalt waterproof coating and a preparation method thereof. The waterproof coating is prepared from the raw materials in parts by weight: 30-60 parts of petroleum asphalt, 10-30 parts of a softener, 3-10 parts of a modifier SBS, 5-20parts of a tackifier, 5-20 parts of waste rubber powder, 5-25 parts of a filler, 3-10 parts of a flame retardant and 2-6 parts of a flowability-increasing agent. The waterproof coating provided by the invention has multiple characteristics such as water resistance, viscosity reduction, easy flowing, flame retardance and the like, the construction temperature and energy consumption are reduced, the non-cured rubber asphalt waterproof coating is prevented from fire in construction and application, and the safety of the non-cured rubber asphalt waterproof coating is enhanced. According to the preparation method provided by the invention, the dissolution difficulty of the modifier in the mixture is lowered, on one hand, the dissolution of the modifier is facilitated, and on the other hand, the preparation temperature of the non-cured rubber asphalt waterproof coating is lowered, and the energy is saved.