Background Chemotherapy side effects can easily contribute to malnutrition and disrupt the normal diet of breast cancer patients. Offering early multidisciplinary team interventions during chemotherapy can establish a solid groundwork for dietary management and enhance the quality of life throughout the survival period. This study aims to assess the impact of early multidisciplinary team interventions on dietary management behavior, self-care self-efficacy, quality of life, and body mass index in breast cancer patients undergoing chemotherapy. Methods A prospective, two-arm, single-blind, single-center randomized controlled trial was conducted between November 2023 and July 2024 from a tertiary-level general hospital in Shaanxi, China. A total of 88 participants who were either preparing for or undergoing early or middle stage chemotherapy were enrolled for this intervention. The intervention group received diet-related early multidisciplinary team interventions, in addition to the usual dietary education. The control group only received the usual dietary education. The intervention program included 8 themes, which were covered each week. The data on dietary management behavior, self-care self-efficacy, quality of life, and body mass index were measured at baseline (T0), immediately after the intervention (T1), 1 month after (T2), and 3 months after (T3) the intervention. Results Seventy-nine participants, divided into an intervention group of 40 and a control group of 39, completed all the measures. There were statistically significant intergroup effects between the two groups and time effects on dietary management behavior, self-care self-efficacy, and quality of life. Additionally, there was an interaction effect (P < 0.05). However, there was no statistically significant intergroup effect on body mass index before and after the intervention (P > 0.05). Conclusion The early multidisciplinary team interventions are an effective method for improving dietary management behavior and confidence among breast cancer patients undergoing chemotherapy. Nurses should be attentive to the dietary issues faced by female patients during chemotherapy and should work to train and organize a multidisciplinary team to provide this intervention. This may lay a theoretical and capable foundation for managing a healthy lifestyle for future survival period. Trial registration This intervention was registered with the Chinese Clinical Trials Registry (ChiCTR2300076503, October 10, 2023).