Objective: To explore the impact of commercial health insurance on the expenditure of basic medical insurance fund. Methods: Based on the panel data of 31 provinces in China from 2012 to 2019，the systematic GMM model and tool variable method were applied in the test. Results: Commercial health insurance significantly reduced the expenditure of basic medical insurance fund at the level of 10 %，and the impact effect showed regional differences ranged by eastern，central and western regions. Conclusion: Commercial health insurance is conducive to sharing the expenditure pressure of the basic medical insurance fund. The impact mechanism may be related to the protection level of commercial health insurance and the consumption tendency of patients.