Background <p>In January 2019, the Chinese government launched a pilot National Volume-Based Procurement (NVBP) program, which was then gradually extended to the whole country to alleviate the financial burden of healthcare expenditure. This study aims to investigate the impact of the NVBP policy on healthcare expenditure.</p> Method <p>By extracting outpatient payment records of breast cancer patients in a tertiary hospital information system from May 2020 to April 2023, we performed an interrupted time series (ITS) to assess the impact of the NVBP policy on healthcare expenditure.</p> Results <p>ITS analyses showed that the NVBP policy mitigated the persistent increase in total average monthly per-visit expenditure, shifting it from an upward trend (9.68 CNY (<i>p</i> &lt; 0.01)) to a flat trend (-2.33 CNY (<i>p</i> = 0.673)). The momentum of the continuous increase in the average monthly per-visit drug expenditure of patients has been effectively curbed; the expenditure has turned from an upward trend (15.10 CNY (<i>p</i> &lt; 0.01)) to a downward trend (-11.19 CNY (<i>p</i> &lt; 0.01)). Although the trend of continuous increase in average monthly per-visit expenditures for chemotherapy drugs (<i>p</i> &lt; 0.01), oral endocrine drugs (<i>p</i> &lt; 0.01) and Traditional Chinese Medicines (TCM) (<i>p</i> &lt; 0.01) were effectively mitigated, especially for TCM; however, the expenditure of injectable endocrine drugs (<i>p</i> = 0.038), examination (<i>p</i> = 0.015) and laboratory (<i>p</i> = 0.029) were the persistent increases.</p> Conclusions <p>Our study suggested that the NVBP policy effectively prevented the continued growth in drug expenditure for breast cancer outpatients, but did not substantially improve total expenditure. In addition, the policy has not only affected the expenditures of medicines, but has also changed the structure of expenditures.</p>