The invention relates to a preparation method of cationic dyeable polyester fiber, first, dimethyl isophthalate-5-benzenesulfonate sodium, ethylene glycol and a catalyst are mixed for ester exchange reaction, after completion of the reaction, ethylene glycol, a complex ion regulator and sulfuric acid are added for adjusting, terephthalic acid, ethylene glycol, a catalyst and a flatting agent are mixed for esterification reaction, a product obtained by adjusting after the ester exchange reaction is added into the reaction system with esterification reaction end point is reached for polycondensation to obtain spinning melt, and finally the cationic dyeable polyester fiber is obtained by metering, extrusion, cooling, oiling, stretching, heat setting and winding of the spinning melt. The method reduces the production of impurities in the fiber production process, thereby reducing the pressure rise of a spinning assembly and a filter, prolonging the service life, reducing the production cost and ensuring the continuity and safety of the whole production process, and the method has good economic value and popularization value.