Abstract Background The present study aimed to develop and test an extended theory of planned behavior (TPB), which includes attitudes, subjective norms and perceived behavioral control, oral health knowledge, and past oral health behavior on the intention to improve oral health behaviors among primary school students in Shanghai, China. Methods A school-based cross-sectional study was conducted with 414 students in the third-grade from 10 classes of Mingqiang Primary School located in Shanghai, China. Participants were recruited in October 2019. Data were collected through self-reported questionnaires, consisting of demographic characteristics, TPB variables, oral health knowledge and past oral health behaviors. Exploratory factor analysis was used to analyze TPB items. Pearson’s correlation and hierarchical regression analyses were conducted to identify the associated factors of intention to improve oral health behaviors. Results The study showed that among students in the third grade, attitudes, subjective norms, perceived behavioral control, and past oral health behaviors were associated with the intention to improve oral health behaviors. In the hierarchical regression analysis, age and sex were entered in Model 1 which significantly explained 3.00% of the variance (F = 6.26, p < 0.01). The addition of Model 2 variables of attitudes, perceived behavioral control, subjective norms, and oral health knowledge revealed that TPB variables explained 26.70% (F = 29.59, p < 0.01). For Model 3, the addition of past oral health behaviors accounted for a further 1% of variance, and the full model has accounted for 28.30% of the variance with the intention to improve oral health behaviors (F = 22.8, p < 0.01). Regression analyses supported that among the significant variables, perceived behavioral control had the largest beta weight, followed by subjective norms and past oral health behaviors. Conclusion The extended TPB model constructed in this study could be used to explain children’s intentions to improve oral health behaviors. Children’s oral health-related perceived behavioral control, subjective norms, and past oral health behaviors may serve as priority intervention targets in oral health promotion practices aimed at children.