The invention relates to a pretreatment method for on-line improving the yarn sizing performance in a sizing machine, belonging to the textile processing field. The technical proposal of the method is that chemical pretreatments of chemical solution soakage of the warp yarns, catalytic reaction of the warp yarns, and the drying treatment of the warp yarns are carried out on the warp yarns before entering into a size box by a yarn guide roller. By adopting the technical proposal of the method, full chemical reaction happens between chemical agent and the warp yarn, thus increasing the combining capacity of the sizing agent; meanwhile, after the warp yarns are dried, solid containing rate of serum in the size box is not affected; in addition, the technical proposal has the function of improving the osmotic effect of the serum in the prewetting of the warp yarns.