The invention discloses a leather finishing agent with color changing function and strong downy feeling and a preparation method thereof, relating to a surface treating agent for synthetic leather. The preparation method comprises the following steps: a, adding a solvent, adding cellulose acetate with stirring, after completely dissolving, adding wax powder to uniformly disperse, then filtering with a 200 mesh sieve, and then putting in a vat; and b, adding the rest materials in order, then stirring to uniformly disperse, wherein the order of adding the materials is a dispersant, a levelling agent, resin, thickening powder, and fluff powder, the stirring speed is 800-1500 rpm, and the temperature is controlled within 50 DEG C; c, after stirring, grinding one to three times by a three-roller machine, and packaging after passing the product inspection to obtain a finished product. According to the invention, the problem of bad dispersibility of fluff powder and wax powder in a high viscosity system is solved, and the prepared finished product has the advantages of good downy feeling, good suede effect, good resilience, good color-change property, high temperature resistance, and strong leather feeling.