The invention discloses a method with functions of preheating and postheating for forming crack-free coating with high efficiency by a three-light-beam laser-cladding technique. The method comprises the following steps of: splitting a laser beam emitted by an Nd: YAG laser into two laser beams by using a laser beam splitter, namely a preheating laser beam for preheating the surface of a base material and a postheating laser beam for postheating the formed coating; then blowing alloy powder into a molten pool which is formed by focusing a laser beam emitted by a CO2 laser and acting the focused laser beam on the surface of the base material by using a powder nozzle, wherein after the CO2 laser beam moves away, a molten layer is cured and crystallized quickly to form the coating; and postheating the formed coating by adopting the postheating laser beam. The method has the advantages that: (1) the dilution rate of the coating is low and adjustable, and the coating is metallurgically combined with the base material, so the base material has a small thermal influence area and is deformation-free and crack-free; (2) residual inner stress in the coating can be eliminated effectively, a tissue can be improved, and the coating has high abrasion resistance, high corrosion resistance, high anti-cracking performance and high thermal shock resistance; and (3) relative to the processing efficiency in the conventional laser cladding technology, the processing efficiency in the method can be improved by 50 times to the maximum extent, so the processing cost is reduced greatly, and a large-scale industrialized application potential is realized.