The invention discloses a soaping agent for printing natural silk fabric with reactive dye and a preparation method of the soaping agent. The soaping agent comprises the following components of a coconut oil triethanol acylamino group surfactant, isomeric undecylic alcohol ethoxylates, octaphenyl polyoxyethyiene, dodecyl polyglucosides, polyvinylpyrrolidone, sodium polyacrylate, chitosan, sodium carbonate, ethanediol and a silicone defoaming agent. When the soaping agent is used for finishing natural silk fabrics, high-temperature treatment is not needed, the natural silk fibers are protected from being damaged, the soaping effect is good, and the stain resistance is high. The soaping agent can ensure that the natural silk fabrics printed with the reactive dye have high firmness, and besides, the condition that washed substances are contaminated for the second time is avoided.