The invention relates to the technical field of polyester filaments, in particular to a preparation method of cationic dyeable polyester filaments. The preparation method comprises the following steps: firstly, adding ethylene glycol and an anti-ether agent to a reactor for stirring, and adding isophthalic acid-5-sodium benzene sulfonate after heating for a direct esterification reaction, judgingthe reaction endpoint on the basis of yield of esterified water, and adding ethylene glycol and sodium acetate trihydrate after the reaction for blending to prepare a solution of SIPE (sodium 5-sulfodimethyl isophthalate), namely, a modified monomer solution; adding the solution of modified monomers to an oligomer, performing pre-polycondensation and final polycondensation reaction to obtain modified polyester melt, and preparing the cationic dyeable polyester filaments with a spinning process. The preparation method has short flow and few procedures, cannot produce toxic, flammable and explosive methanol in the process of preparing the modified monomer solution, and is environmentally friendly; besides, no heavy metal catalyst is used in the reaction process, so that the cost is saved, and the risk of harm to the environment and human body is reduced.