The invention discloses flame-retarding lacquer putty and a preparation method thereof, and aims to provide air-dried organic silicon-modified unsaturated flame-retarding lacquer putty which has good comprehensive performance, long-term use at the temperature of below 250 DEG C and self-drying property at normal temperature to meet special requirements of subway vehicles and casthouse vehicles, wheel shafts, smoke (steam) passages and the like in other high temperature places. The air-dried organic silicon-modified unsaturated flame-retarding lacquer putty is characterized by being prepared by mixing raw material nano-montmorillonite not containing a halogen element compound, reactive diluent, a heat-resistant unsaturated polyester resin, an organic silicon resin, a special filler, a flame retardant, a low shrinkage additive, a stabilizer, dimethylaniline, a dispersant, titanium dioxide, hollow glass micro-beads, a drier, molybdenum disulfide, talcum powder, ketone peroxide, a curing agent filler, quinacridone bluish red and polyvinylpyrrolidone in a certain proportion, dispersing and grinding the mixture to prepare a main agent and a curing agent, and mixing the main agent with the curing agent. The flame-retarding lacquer putty is mainly used as a bottom coating material for subway vehicles.