Yak milk has nutritional properties of high protein and high fat content, and has good functional activities, such as anti-fatigue. However, due to the low production of yak milk and the scattered production area, there was limited research on the development of cheese products and other deep processing aspects. Penicillium roqueforti (P.roqueforti) was used as a secondary starter to develop blue style-yak milk cheese, and its ripening characteristics and flavor characteristics were analyzed. Results showed that P. roqueforti could grow rapidly in yak milk cheese, promoted the degradation of proteins, and formed a large number of peptides, amino acids, etc. The content of free amino acids, especially unami amino acids, was significantly increased at the end of yak milk cheese maturity (P＜0.05). SPME-GC/MS analysis showed that a total of 53 flavor components were detected in yak milk cheese. OPLS-DA analysis revealed that compared with the early stage of ripening, the composition of flavor components in yak milk cheese at the end of ripening was significantly different, and the total content was significantly increased (P＜0.05). The volatile aliphatic esters in yak milk cheese were mainly methyl esters, and the content was relatively stable in end stage (25d, 46d) of ripening. The content of volatile fatty acid was highest in the middle stage of ripening, but significantly decreased at the end of ripening (P＜0.05). The concentration of volatile aliphatic ketones, especially methyl ketones, increased significantly in the end ripening stages (P＜0.05). 2-Nonanone, 2-heptane, methyl octanoate, 2-pentanone, ethyl caprate, methyl caprate, caproic acid, 2-octanone, butyric acid and other volatile compounds were quantitatively dominant flavor components of blue style-yak milk cheese. The study showed that P. roqueforti could promote the ripening of yak milk cheese, which could significantly affect the nutrition and flavor characteristics of yak milk cheese. The research aimed to lay a theoretical foundation for the deep processing of characteristic dairy products such as yak milk in China, and provide reference for promoting the development of China's characteristic dairy deep processing industry.