This work studied the inulin which is a kind of inulin-type fructans with high degree of polymerization and the oligofructose which is a kind of inulin-type fructans with low degree of polymerization at different concentrations on the quality of fermented milk containing Bifidobacterium BB-12. Based on the theory of Diffusing Wave Spectroscopy （DWS）， the changes in macroscopic viscosity index of fermented milk with the two kinds of inulin-type fructans was analysis， and the quality of inulin and oligofructose fermented milk was evaluated on the fermented time， the viable count of Bifidobacterium BB-12 and all Lactobacillus， titratable acidity， precipitate amount of whey and viscosity of fermented milk during storage. The result showed that inulin and oligofructose affected the progress of milk turning into a gel and they extent the fermented time. Additionally， during the storage time， inulin and oligofructose had an ability to decrease the decline of viable count of Lactobacillus， increase the viscosity， reduce the precipitate amount of whey and titratable acidity. Thus， they had improved the quality of fermented milk， and inulin had a better effect on it compared with oligofructose， but they may have different mechanisms on it， which reminds further study in microbial metabolism and interactions with protein.