Based on the data of M&A of China's listed companies from 2010 to 2018, this paper studies the ex ante selection effect of financing constraint on M&A and the ex post treatment effect of M&A on target's financing constraint by using Logistic regression and Difference-in-Difference (DID)estimation. The results show that: First, firms with advantages in financing capability are more likely to become potential acquirers, and they prefer medium-sized firms with differences in the degree of financing constraint from them (higher than themselves); Second, after M&A, the degree of target's financing constraint is significantly reduced in general but there are heterogeneity in targets with different growth capabilities and ownership.For non-state-owned enterprises and enterprises with higher growth capability, the mitigation effect of financing constraint brought by M&A is more significant. This research result shows that it is an important driving force for M&A to obtain synergy effect by alleviating the target financing constraints.