The invention belongs to the technical field of magnetic resonance imaging (MRI) contrast agents, and discloses a preparation method of a sericin-gadolinium pH responsive targeting tumor nuclear magnetic resonance contrast agent. The contrast agent is used for enhancing MRI research of tumor tissues, sericin, gadolinium acetate hexahydrate and gadolinium chloride hexahydrate are adopted as raw materials, and the nano contrast agent SS@GAH-GdCl3 is synthesized through a Schiff base reaction. Amino groups of the sericin and aldehyde groups of the gadolinium acetate hexahydrate are subjected to aone-step reaction by a two-step method to form Schiff base, and gadolinium ions are supplemented through electrostatic adsorption of the gadolinium chloride hexahydrate. The contrast agent prepared by the method can smoothly pass through normal tissues and blood vessels, and the surface potential of the contrast agent can be automatically reversed at a tumor part to enter tumor tissues, so that the uptake of tumor cells is increased, and the precise MRI contrast of solid tumors is realized; and the metabolism time of the nano contrast agent is remarkably prolonged by 30-60 min, and is far longer than the pharmacokinetic time of a commercial gadodiamide injection.