A method of preparation of brominated polystyrene combustion inhibitor with low molecular weight by active/controllable polymerization. Brominated polystyrene with controllabel low molecular weight is prepared by atom transfer radical polymerization (ATRP) or reverse atom transfer radical polymerization (FATRP), comprising using organic halogen as initiator, adding azo-initiator, catalyst of low valent metal halogen and ligand into FATRP, then obtaining narrow distributed brominated polystyrene with low molecular weight by bulk polymerization, suspension polymerization, emulsion polymerization. The polystyrene is mixed with alkyl halide, alkyl aluminium halide, dehydrant, brominated by bromine to get narrow distributed brominated polystyrene with low molecular weight. Polystyrene or brominated polystyrene of different molecular weight can be respectively obtained by controlling weight of added monomer or bromine. The obtained brominated polystyrene combustion inhibitor with low molecular weight has high content of bromine, good compatibility with matrix, great heat durability and ageing resistance, little influence on impact resistance of basic resin.