Based on the theory of financial flexibility, the paper discusses the effect of financial flexibility (including cash flexibility and debt flexibility) on the relationship between free cash flow and under-investment, and uses the samples of Chinese listed companies to carry out empirical analysis. The results show that the level of free cash flow is the restraint to the under-investment behavior; both cash flexibility and debt flexibility have a significant positive regulatory effect on the relationship between free cash flow and under-investment; from the perspective of different sources of debt flexibility, remaining short-term bank borrowing capacity has a significant positive regulatory effect on the relationship between the free cash flow and under-investment, but remaining commercial credit capacity has not a significant effect on the relationship.