Polysaccharides of Phellinus igniarius have many bioactivities. In this study, response surface methodology along with Box–Behnken design based on the single-factor experiments was firstly applied to optimize the extraction conditions by its liquid-cultured mycelia. According to analysis, extraction temperature and ratio of mycelia to water significantly affected extraction yield. The optimal conditions were extraction temperature 70 °C, extraction time 1.5 h and the ratio of mycelia to water 1:6.2. Under these conditions, the maximal yield of crude intracellular polysaccharide (IPS) from mycelia was 50.39 ± 0.41 mg/g, which was agreed with model predictions. The preliminary characterization was mainly β-galactan by gas chromatograph and infrared spectrum analysis.