The invention discloses a method for assisting in evaluating birth weight and weaning weight of piglets and a special primer of the method. The invention provides applications of polymorphism or genotype materials to preparation of products, the polymorphism or genotype materials are used for identifying a plurality of loci in a CNN3 genome of the piglets to be detected, and the loci include No.182 nucleotide of second intron, No.44 nucleotide of third intron and No.314 nucleotide of fourth intron. The applications of the products includes assisting in identifying the birth weight of the piglets to be detected; assisting in evaluating the birth weight of the piglets to be detected; assisting in comparing the birth weight of different piglets to be detected; assisting in identifying the weaning weight of the piglets to be detected; assisting in evaluating the weaning weight of the piglets to be detected; and assisting in comparing the weaning weight of the different piglets to be detected. According to the method and the special primer, mutation of the three loci in a deoxyribose nucleic acid (DNA) sequence of the CNN3 genome of the piglets is disclosed, a technology for detecting growth performances of the birth weight and 21-day-old weaning weight of the piglets at a single nucleotide polymorphism (SNP) locus is provided, and a new mark is provided for mark assistant breeding of the piglets.