Naturally-fermented mixed fruit products are popular in Asia for their abundant nutrition, potential antioxidant activity, and hypoglycemic activity. In this work, the changes in chemical composition (including pH, titratable acid, total and reducing sugar, total soluble solids, protein content, total phenols, and organic acids) and preliminary in vitro bioactivities (including ABTS radical scavenging activity, DPPH radical scavenging activity, α-amylase, and α-glucosidase inhibition) of naturally-fermented mixed fruit products during a 120-day fermentation period and their correlations were investigated. Results showed that a variety of organic acids changed dramatically, such that lactic acid content raised from 0 mg/mL at 0 day to 9.11 mg/mL at 60 days; and citric acid content raised from 3.38 mg/mL at 0 day to 6.52 mg/mL at 90 days. Correlation analysis indicated that total phenols, citric acid, lactic acid, and titratable acid were the key factors influencing the continuous increase of ABTS, DPPH radical scavenging, and α-amylase inhibition during fermentation. In general, natural fermentation provides a great increase in the nutritional potential and functional value of the fruit and the results of this study provide evidence for the scientific guide in the industrial production of fermented fruit. •Lactic acid and citric acid continued to rise in the first 60 days of fermentation.•Functional activity is enhanced by the presence of phenols and organic acids.•Natural fermentation increases the functional value of the fruits considerably.