Abstract: Skim yak milk as the research material, titratable acidity as review indicator, with three strong fermentability lactic acid bacteria including Lactobacillus delbrueckii subsp bulgaricus (MGD1-3) , Streptococcus thermophilus (MGB39-5) , Lactobacillus plantarum (BM5152) were researched and selected the best inoculation amount in skim yak milk. Three parameters were optimized by response surface central rotating composite design for process optimization. Finally, the mathematical model of each parameter was set up and these response values were analysed. The result showed that the influencing factors on titratable acidity of skim yak milk were MGD1-3>MGB39-5>BM5152 at 42℃ and fermentation lasted for 3.5h. The best inoculation amount of MGD1-3, MGB39-5, BM5152 was 5.29, 7.18, 6.30lgcfu/mL respectively, and the titratable acidity was 71.90°T.Inoculated equal optimized combination starter and bacteria-containing whey liquid from pastoral areas to verify test, the result showed that the fermentation time significantly was decreased to 8.7h (p<0.01) , titratable acidity went up to 3.97°T (p<0.05) and the viable count significantly was increased to 7.9×107cfu/mL (p<0.01) .