The invention relates to high-efficiency heat-insulation environmentally-friendly flame-retardant modified polyolefin indoor heat-insulation foam and a preparation method thereof. The preparation method comprises the following steps of: kneading low-density polyethylene resin, a composite flame retardant, an anti-smoke agent, an antioxidant and a lubricant at normal temperature, then discharging to an internal mixer for internal mixing, then discharging to a single-screw extrusion granulator, and carrying out melting, plastication and granulation to obtain a master batch A; adding low-density polyethylene resin to a high-speed mixer, then adding azobisformamide, talc powder, a lubricant and zinc stearate, kneading at normal temperature, and then discharging to a single-screw extruder for granulating to obtain a master batch D; adding low-density polyethylene resin, ethylene-vinyl acetate copolymer, ethylene propylene terpolymer, the master batch A, a master batch B, hollow ceramic microbeads, a rheologic agent and a lubricant to a high-speed mixer, kneading at normal temperature for 3 minutes, then discharging to a single-screw extruder, and extruding into sheets; carrying out radiation cross-linking on the extruded sheets through an electronic accelerator; placing the cross-linked sheets in a foaming furnace for foaming; and coating expansible fireproof paint on the surface of the foam.