The invention discloses low viscosity flame-retardant room temperature vulcanized foam silicone rubber and a preparation method thereof. The silicone rubber comprises a component A and a component B in a mass ratio of 1: 1. The component A comprises 100 parts of alpha, omega- dihydroxy polydimethylsiloxane, 40-60 parts of a flame retardant A, 10-30 parts of a reinforcing and incrementing filler, 10-30 parts of methyl phenyl silicone oil, 15-30 parts of vinyl end silicone oil, 0.4-0.8 part of a Pt catalyst and 5-10 parts of a flame retardant B; the component B comprises 100 parts of alpha, omega- dihydroxy polydimethylsiloxane, 5-10 parts of end hydroxyl silicone oil, 40-80 parts of a flame retardant A, 15-35 parts of a reinforcing and incrementing filler, 10-20 parts of methyl phenyl silicone oil, 15-30 parts of vinyl silicone oil, 10-20 parts of hydrogen-containing silicone oil and 5-10 parts of a flame retardant B. The foam silicone rubber disclosed by the invention is low in viscosity and easy to flow and is easily cured at constant temperature, and is applied to pouring construction conveniently.