The purpose of this study was to evaluate the effect of a bi-annual professional application of acidulated phosphate fluoride (APF) foam on caries increment in the primary dentition over a two-year period in the People's Republic of China. In a double-blind, cluster-randomized, placebocontrolled trial, 392 children aged 3-4 years from 15 classes were randomly assigned to two groups on a school class basis. The experimental group (8 classes) received a bi-annual APF foam application, and the control group (7 classes) received the placebo. The mean increment of dmfs in the experimental group was 24.2% lower than that in the control group (p<0.05). The significant caries reduction was observed on approximal surfaces in the experimental group compared with the control group (p<0.01), but there were no differences on occlusal surfaces (p>0.05). A bi-annual professional application of APF foam was effective in reducing the increment of dental caries in the primary teeth.