Abstract: To explore and develop new resources of Ganoderma lucidum polysaccharides, the Ganoderma sessile was chosen for research, as there has been relatively under-researched and contains a higher content of active ingredients. A microwave-assisted extraction method was employed, using the polysaccharide content of Ganoderma sessile fruiting body as the evaluation index. Single-factor experiments were conducted to examine the effects of five factors—solid-to-liquid ratio, microwave power, extraction time, number of extractions, and soaking time—on the extraction efficiency of Ganoderma polysaccharides. The Plackett-Burman design was applied to identify three significant factors: extraction time, soaking time, and the number of extractions. A three-factor, three-level regression model was then established using the Box-Behnken response surface methodology to analyze the influence of key parameters on polysaccharide content and perform systematic optimization. Finally, using a VC solution as a positive control, the in vitro antioxidant activity of the polysaccharides obtained under the optimal process was studied by determining the total reducing power, DPPH radical, ABTS radical, and OH radical scavenging rates of the polysaccharides within a certain concentration range. The results indicate that the optimal extraction conditions for polysaccharides from Ganoderma sessile fruiting body are a material-to-liquid ratio of 1:30 g/mL, microwave power of 325 W, extraction time of 9 minutes, soaking time of 30 min, with 3 extractions. The polysaccharide content is 26.34%, which is close to the predicted value, and the RSD is less than 0.7%. The optimized extraction process for Ganoderma sessile polysaccharides is simple to operate, has good stability, and is feasible. Moreover, the extracted polysaccharides exhibit strong in vitro antioxidant activity, demonstrating good development potential.