Major customers in strong bargaining position can exert pressure on dependent suppliers and adversely affect their financing conditions. Consistent with this prediction, our analysis shows that the concentration of customer bases can enhance the bargaining power of downstream customers in supplier-customer interactions, leading to the significant deterioration of financing constraints for upstream firms. We then introduce a novel machine-learning approach to analyze the heterogeneous effect of customer concentration. This allows us to identify approximately 15 % of the firms, especially those small non-SOEs, as most vulnerable to customer concentration as the bargaining effect dominates.