The invention relates to a preparation method of a cationic dyeable PTT, the preparation method sequentially comprises the first esterification reaction: 1,3-propanediol and terephthalic acid are added in a reactor according to a certain proportion, an esterification catalyst and an auxiliary agent are simultaneously added, the temperature is increased to 220-260 DEG C, and the reaction is finished till the esterification outlet water; the second esterification reaction: mixed slurry of dicarboxylic acid and 1,3-propanediol is added in the reactor, the temperature is increased to 220-260 DEG C for carrying out the second esterification reaction till the esterification outlet water, a polymerized monomer is obtained after the reaction is finished; the polymerization reaction: the polymerized monomer, the 1,3-propanediol solution of 5-sodium sulfonate dimethyl isophthalate, a flexible chain segment component, a polymerization catalyst and a stabilizer are sequentially added in the polymerization reactor, and the conventional polymerization is carried out under the conditions that the pressure is less than or equal to 40Pa and the temperature is 240-280 DEG C. The cationic dyeable PTT which is obtained by the method simultaneously has good rebound resilience and good cationic dye dyeable function under normal pressure.