To evaluate the in vitro antioxidant activities and carbohydrate digesting enzymes inhibitory activities of polyphenols among four varieties of Cinnamomum cassia Presl (C.cassia) cultivars (China Taiwan soil C.cassia and Qinghua C.cassia, Yunye C.cassia and Xijiang C.cassia), free and conjugated polyphenol contents, free and conjugated flavonoid contents of the four C.cassia cultivars were analyzed. The antioxidant activities and carbohydrate digesting enzymes inhibitory activities in vitro of crude extracts of free and conjugated polyphenols from the four C.cassia cultivars were also determined. Furthermore, correlation analysis of polyphenols and flavonoids with carbohydrate digesting enzymes inhibitory activity were performed. Qualitative and quantitative analysis of polyphenol compounds in the crude extracts of free polyphenols from the four C.cassia cultivars was carried out by UHPLC-MS/MS. The results showed that the contents of total polyphenols and total flavonoids in the four C.cassia cultivars ranged from 58.99-71.62mg/g and 46.49-58.05mg/g, respectively. And the contents of free polyphenols and flavonoids of the four C.cassia cultivars were higher than those of conjugated polyphenols and flavonoids. In addition, significant differences were detected with the antioxidant activity and carbohydrate digesting enzymes inhibitory activity among different cultivars. The antioxidant activity and carbohydrate digesting enzymes inhibitory activity of the China Taiwan soil C.cassia were significantly higher than those of other cultivars (P<0.05). Meanwhile, the results showed that free and conjugated polyphenol contents, free and conjugated flavonoid contents had a negative correlation with IC₅₀ values of α-glucosidase, α-amylase and sucrase (P<0.05). The results obtained by UHPLC-MS/MS suggested that proanthocyanins, quercetin-3-O-glucoside and rutin might be the inhibitors of carbohydrate digesting enzymes in the extracts. The differences of the biological activity of C.cassia among the four cultivars provided a theoretical reference for the variety screening, precise processing and efficient application of C.cassia.