The invention relates to a preparation method of low-molecular-weight halogen flame-retardant polycarbonate, which is characterized by comprising the following steps: a. adding polyhalo bisphenol A into a sodium hydroxide solution-haloalkane mixed system, completely dissolving by stirring at normal temperature, and meanwhile, adding a catalyst; b. introducing phosgene, meanwhile, dropwisely adding a sodium hydroxide solution to keep the pH value of the system at 7-14, and carrying out prepolymerization reaction at 0-20 DEG C for 0.2-1.5 hours; c. increasing the reaction temperature to 10-40 DEG C, adding a blocking agent, and meanwhile, dropwisely adding a sodium hydroxide solution to keep the pH value of the system at 7-14; d. after the reaction finishes, standing to stratify to obtain an organic phase, adding fatty alcohol, washing to obtain a white solid, filtering, and drying to obtain the product. By strictly controlling the prepolymerization and polycondensation reaction temperatures and the pH value of the reaction system, the synthesized product has small fluctuation of the molecular weight, and the molecular weight can be fixed within a certain range. The synthesis technique has the advantages of short period, simple operating technique and high efficiency.