The optimal parameters for ultrasonic-assisted enzymatic extraction of polysaccharides from Potentilla anserina L. using a mixture of cellulase, pectinase and papain (2:1:0.5) were determined as follows: enzyme dosage 49 U/g, ultrasonic time 100 min and pH 4.4. Under these conditions, the maximum yield of polysaccharide extract from Potentilla anserina L. (PAPE) of (6.89 ± 0.08)% was obtained. Furthermore, the effect of PAPE on yak yoghurt fermentation was investigated. The results showed that PAPE could significantly enhance the viable count and acid-producing capacity of Streptococcus thermophilus CICC 6038, Lactobacillus bulgaricus CICC 20247 and Lactobacillus fermentum HY01 in a dose-dependent manner. Yoghurt added with PAPE at 2 g/100 mL concentration had a hardness of (163.0 ± 9.9) g, a viscosity of (-182.9 ± 1.6) g·s, a consistency of (69.7 ± 1.7) g·s, a cohesiveness of (410.0 ± 6.0) g and it had a good taste, with a sensory score of 7.78; its antioxidant capacity in terms of scavenging capacity against hydroxyl, superoxide anion and 2,2’-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) cation radicals increased significantly compared with the control group. This study provides new ideas for improving the quality and added value of yak yogurt.