The present invention discloses ganoderma atrum drink having immunity improving functions and a preparation method thereof. The ganoderma atrum drink comprises the following substances in ratios: 100 mL of ganoderma atrum concentrate supernatant liquid, 0.1-0.3 g of erythritol, 0.001-0.005 g stevioside, 0.02-0.06 g of citric acid, 0.04 g of sodium carboxymethyl cellulose and 0.03 g of sodium erythorbate. The preparation method comprises the following steps: raw material pretreating, ganoderma atrum extract preparing, ganoderma atrum extract concentrating, clarifying, ganoderma atrum drink blending, filling and sterilizing, etc. The preparation method sufficiently develops the health-care values of the ganoderma atrum in improving immunity, can maximally extract ganoderma atrum polysaccharides and maintains the characteristic flavors of the ganoderma atrum. The drink is high in stability and the preparation method maximally reduces the discoloration of the drink in the processing and preserving processes. The process is simple, the equipment requirements are low, and the preparation method is easy to realize industrial production.