The present invention relates to a preparation method for a laser-markable halogen-free fire retardation polyester having a variable ground color, and belongs to the technical field of polymer processing. The halogen-free fire retardation polyester comprises, by weight, 40-87% of polybutylene terephthalate, 0.1-2% of a coloring agent, 0.2-1% of a metal oxide, 0-2% of a silicate mineral, 10-30% of a halogen-free fire retardation agent, 2-10% of a toughening agent, 0.2-1% of an antioxidant, 0.5-1.5% of a processing auxiliary agent, and 0-40% of continuous glass fibers. The preparation method comprises: drying various components, uniformly mixing according to a certain ratio, conveying into a twin-screw extruder, and carrying out melting, mixing, extruding, strip drawing, cooling, drying and palletizing through the extruder to obtain particles. The laser-markable halogen-free fire retardation polyester has characteristics of excellent fire retardation, excellent mechanical property and multiple colors, and presents the black marker with the clear edge and high contrast degree in the laser beam irradiation. The preparation method is simple and is suitable for continuous mass production.