Abstract: Objective：To comparatively analyze the caries activity among early childhood caries, severe early childhood caries and caries free children. Methods：A total of 289 children aged 3-5 years in three kindergartens in Nanjing were examined for dental caries and caries activity test. The parents were investigated and the results were statistically analyzed. Results：The caries prevalence rate of primary teeth was 47.8%, the prevalence of SECC was 20.4%. Cariostat value was significantly correlated with dmft and dmfs, and had statistical differences between the three groups. Sweet food/drink before sleep and Cariostat value was significantly related to the caries. Conclusions：The cariogenic bacteria activity of early childhood caries can reflect the severity of caries.