The invention relates to a synthesis and purification method of carboxymethylated ganoderma lucidum polysaccharide. The method includes the steps of: adding ganoderma lucidum polysaccharide and sodium hydroxide into a solvent to conduct stirring dissolving, then adding chloroacetic acid, carrying out stirring reaction at 65-85DEG C for 2-4h, performing cooling, adjusting the pH value to neutral, conducting alcohol precipitation and filtering, collecting the precipitate, then dissolving the precipitate in water, and carrying out alcohol precipitation, dialysis, and freeze drying to obtain light brown cotton-like carboxymethylated ganoderma lucidum crude polysaccharide. Test proves that: the carboxymethylated ganoderma lucidum polysaccharide provided by the invention has very good physical and sensory moisturizing effects, also can reduce cigarette stimulation and miscellaneous gas, improves the mellow and full sense and comfort of cigarette smoke, and can applied as an excellent performance tobacco humectants in cigarettes.