The invention belongs to the technical field of building materials, and particularly relates to flame-retardant thermal insulation mortar doped with cob cores and a preparation method thereof. The flame-retardant thermal insulation mortar doped with the cob cores is prepared from ordinary portland cement, superfine slag powder, desulphurization gypsum, calcium hydroxide, glass beads, the cob cores, EVA (ethylene-vinyl acetate) emulsion powder, an air entraining agent and hydroxypropyl starch ether and the like, wherein the cob cores serve as main lightweight aggregates. The flame-retardant thermal insulation mortar is prepared by fully utilizing the agricultural waste cob cores and other solid wastes, low in production cost, simple and fast in construction, safe, durable and non-combustible and has good construction performance and mechanical property, excellent heat preservation property and fireproof performance, A-grade fire resistance and good economic benefits and social benefits.