The invention provides ganoderma lucidum polysaccharide of high molecular weight and a detection method thereof. The ganoderma lucidum polysaccharide of high molecular weight is obtained via water extraction, alcohol precipitation, purification, and freeze-drying. The preparation method of the ganoderma lucidum polysaccharide of high molecular weight is simple and is suitable for large-scale production. The polysaccharide content is more than 65 percent, and macrophages can be stimulated to produce nitric oxide (NO).