This study developed a lactic-acid beverage of Ganoderma lucidum fermented soybean liquid, and used DMAIC to improve soybean fermentation with Ganoderma lucidum. The Taguchi method was utilized to determine the important factors for improvement. The range of parameters (inoculation amount, dissolved oxygen content, and temperature) greatly affecting the process served as reference for controlled factor levels in the Taguchi method. The Taguchi method analyzed the contribution rates of various control parameters and obtained the optimal control factor combinations, with the highest Polysaccharides target value. The inoculation amount was 6%, dissolved oxygen content was 90%, and temperature was 30°C. By using the same fixed factors, Ganoderma lucidum Polysaccharides was 2.59-4.56 g/kg and 4.46-4.63 g/kg, before and after improvement, respectively. The process can increase Polysaccharides yield from Ganoderma lucidum and stabilize the yield rate.