The invention discloses water-soluble radix puerariae fiber oligosaccharide and a preparing method thereof. The preparing method includes the steps that 1, radix puerariae residues are adopted as a raw material, water is added, and the radix puerariae residues are smashed in a stirred mode to obtain radix puerariae residue pulp; 2, the radix puerariae residue pulp is placed in an electric heating oil bath pan for steam precooking, and radix puerariae fiber residues are obtained through suction filtration; 3, the radix puerariae fiber residues are prepared into radix puerariae fiber residue pulp; 4, the radix puerariae fiber residue pulp is subjected to enzyme hydrolysis for two times and treated ultrasonically and supplementarily to obtain radix puerariae fiber hydrolysate; 5, membrane separation is carried out on the radix puerariae fiber hydrolysate obtained after enzyme hydrolysis is conducted for two times, fiber oligosaccharide with the polymerization degree of 2-5 is enriched, and the good-water-solubility fiber oligosaccharide with the polymerization degree of 2-5 can be prepared. The water-soluble radix puerariae fiber oligosaccharide can serve as prebiotics to be applied to related functional food and medicine, new resources are developed, and a new way is provided for full utilization of radix puerariae.