The invention provides a goldfish fin color changing method comprising the following steps: catching the goldfish from a fish tank; using a sponge to absorb the water on the surface of the fin; using a non-woven fabric to wrap the fin; evenly dripping bamboo vinegar weak solution on the wrapping non-woven fabric, and contentiously wrapping for 1-1.5 minutes; removing the wrapping non-woven fabric, placing the goldfish into a water tank with clean water, allowing the goldfish to freely swim for 1 minute, catching the goldfish, and flatly placing the goldfish on a work table with a pre-laid sponge; using a cotton swab to dip a color changing agent, evenly coating the agent on the selected goldfish fins, and solidifying for 1-2 minutes after coating until the color changing agent solidifies to form a stable color changing film. The method can change colors for goldfish fins according to a goldfish color changing scheme; the color changing agent is suitable for live goldfishes, can change the fin colors without affecting live goldfish normal life and growth, and the goldfish living water environment cannot be polluted.