Using the data of A-share enterprises in the years between 2004 and 2012, this study investigates the relationship among the fluctuation of monetary policies, internal control, and financial constraints. The results show that although financial constraints are common among the firms, financial constraints tend to be strengthened by monetary contractions, and internal control could mitigate the positive effect of monetary contractions of financial constraints. After distinguishing between firms in different external environments and internal characteristics, we find that under monetary contractions, internal control could have greater mitigating effect on financial constraints in the enterprises which are located in areas with low degree of marketization and in private enterprises than in the enterprises which are located in areas with high degree of marketization and in state-owned enterprises. This study confers a new perspective on the determinants of financial constraints, provides evidence for the credit transmit channel of monetary policy, and gives empirical support to the internal control building of companies in our country.