The invention relates to a preparation method and application of a dyeing promoter. The invention aims to provide a method for preparing a promoter, which can carry out cation modification on natural fibers like cotton and silk, and provides a technology for modification of natural fibers by employing the above mentioned dyeing promoter. A technical scheme is as below: a preparation method of a polypichlorohydrin amide dyeing promoter is carried out according to the following steps: 1) synthesis of polypichlorohydrin; and 2) synthesis of the polypichlorohydrin amide dyeing promoter. The method for modification of natural fibers by employing the polypichlorohydrin amide dyeing promoter is carried out according to the following steps: preparing a modification liquid according to the scheme, and placing the modification liquid in a constant temperature water-bath pan for oscillation; and adding a natural cellulose fabric into the modification liquid, and taking out and washing the fabric; or preparing a modification liquid according to the scheme, and placing the modification liquid in a constant temperature water-bath pan for oscillation; and adding a natural silk fabric into the modification liquid, and taking out and washing the fabric.