The invention relates to a method for synthesizing a coenzyme Q10 from a 5-demethoxy coenzyme Q10. The method comprises the following steps of: taking the 5-demethoxy coenzyme Q10 as a raw material, and carrying out a 1,4-addition reaction, a methoxylation reaction and an oxidation reaction to finally prepare the coenzyme Q10. According to the method, the 5-demethoxy coenzyme Q10 is used as a rawmaterial, the process route is short, the yield is high, the product purity is high, the content detected by adopting HPLC (High Performance Liquid Chromatography) of a pharmacopoeia analysis method is 98% or more, and the problem of a byproduct 5-demethoxy coenzyme Q10 generated in a production fermentation process of the coenzyme Q10 is solved, thereby changing wastes into valuables.