Objective: To analyze the relationship between patient expenses and basic medical insurance fund expenditure in tertiary public hospitals. Methods: Based on the monthly data of basic medical insurance and medical expenses published by the National Health Commission and the National Medical Security Administration from 2019 to 2021，a Vector Auto Regression (VAR) model was established，and the impulse response function was used to analyze the relationship between these two variables. Results: There is a two-way Granger causality between the patient expenses of tertiary public hospitals and the expenditure of basic medical insurance fund. The impact of the former on the latter can last to 8 periods，while the impact of the latter on the former is mainly concentrated in the first 4 periods. Conclusion: The patient expenses and basic medical insurance fund expenses in tertiary public hospitals influence each other，and respond after being impacted by each other. The impact intensity of different variables is different，but the impact of the former on the latter lasts longer.