Pregnant women in China are at risk of malnutrition due to an interplay of physiological, psychosocial, and environmental factors. Understanding the factors influencing dietary behaviors in pregnant women is vital for promoting maternal and infant health. This study explores the determinants of dietary behaviors among pregnant women in China based on the Theory of Planned Behavior (TPB).A cross-sectional survey was conducted with 733 pregnant women recruited from a hospital in Beijing, China, between May 2023 and June 2024. Participants completed a questionnaire to assess dietary patterns and psychosocial factors influencing dietary behaviors. One-way ANOVA analysis was used to test differences among dietary patterns in pregnant women. Multiple logistic regression analysis was conducted to explore the associations between the determinants of the TPB model and dietary behaviors. Decision tree analysis was used to determine cut-off values and assess the importance of determinants affecting dietary behaviors.Factor analysis identified 3 dietary patterns: balanced dietary, unhealthy snacks, and healthy snacks. Multivariate analysis showed that after adjusting for age, education, number of pregnancies, and Pre-pregnancy Body Mass Index (Pre-BMI), pregnant women who report higher environmental scores (OR = 1.55, 95% CI: 1.22-1.97) and lower self-efficacy scores (OR = 1.51, 95% CI: 1.05-2.15) are at an increased risk of adopting unhealthy snack patterns. Conversely, higher positive emotion and self-efficacy scores reduce the risk of adopting unhealthy snacking patterns and make it more likely to adopt a balanced diet pattern(OR = 0.69, 95% CI: 0.55-0.86; OR = 0.56, 95% CI: 0.36-0.89). Finally, we ranked the importance through the decision tree, and the results from high to low were positive emotions, self-efficacy, age, and Pre-BMI.The study suggests interventions focused on enhancing attitudes, self-efficacy, particularly in fat intake management and food label reading, could promote healthier eating behaviors among pregnant women. Future research should employ longitudinal designs to confirm these associations and inform effective nutrition strategies tailored to pregnant women in China.This study was preregistered on May 5, 2023, at the Chinese Clinical Trial Registry (ChiCTR2300071126).© 2025. The Author(s).