The invention discloses a method for preparing linear polydichlorophosphazene by means of catalytic synthesis. The method comprises the steps of dissolving hexachlorocyclotriphosphazene into a high boiling point chlorinated solvent, heating up to 200-215 DEG C under the protection of inert gas, and carrying out a reaction for 3-10h by taking fuming sulphuric acid as a catalyst and ultrapure wateras a synergistic catalyst; after the reaction is finished, pouring reaction mixed liquid into n-heptane for separating out the product linear polydichlorophosphazene, and then washing the product withthe n-heptane for a plurality of times so as to obtain the pure linear polydichlorophosphazene. The catalyst used in the synthesis method introduces less impurities, and is high in utilization rate,easy to disperse and high in catalytic efficiency; the synthesis method is easy in control of reaction temperature and lower in requirements for heating rate and reaction temperature, thus being suitable for industrial scale production; the linear polydichlorophosphazene obtained by the method has a yield of 45-70%, a number average molecular weight Mn of 50,000 or more, and a molecular weight distribution coefficient of 1.08-1.20.