The impact of adding polysaccharides extracted from liquid-cultured mycelia of Inonotus obliquus into yak yogurt on its sensory evaluation score and titratable acidity was investigated to determine the optimal addition amount of the polysaccharides. The process parameters for producing yak yogurt with added Inonotus obliquus polysaccharides were optimized, and the quality of the yogurt prepared under optimized conditions was evaluated and compared with that of yogurt without added Inonotus obliquus polysaccharides as a control. The results showed that addition of 0.4% Inonotus obliquus polysaccharides to yak yogurt led to the highest sensory score together with a moderate acidity. Using orthogonal array design, the optimal process conditions were determined as follows: ratio of Streptococcus thermophilus G2 to Lactobacillus paracasei L9, 1:1; fermentation time, 14 h; temperature, 38 ℃; and sucrose concentration, 8 g/100 mL. Compared with the control, the as-prepared yogurt increased the number of viable lactic acid bacteria, titratable acidity and water-holding capacity by 34.1%, 11.0% and 16.8% respectively, and the pH value decreased accordingly. Addition of Inonotus obliquus polysaccharides into yak yogurt improved its firmness and viscosity as well as flavor.