Based on the information data of firms' top five customers and suppliers listed in A-share from 2009 to 2021, this paper explores the influence and mechanism of major customer as supplier on corporate financing constraints. We document that major customer as supplier aggravates firms' financing constraints, and this effect is more pronounced for firms owned by the non-state and located in non-manufacturing industry as well as for customers whose sales ratio is greater than procurement ratio and located in the same industry with firms. Mechanism test shows that related-party transactions are the channels in which major customer as supplier aggravates firms' financing constraints, rather than the financial frauds as well as the weak market position and high operating efficiency brought by real business mode, suggesting that major customer as major supplier is a signal of supply chain risk in the Chinese context and related-party transactions are often hidden behind it. This paper expands the academic and practical circles' understanding of firms'major customers and suppliers, and also provides a new explanation for the high financing constraints in China from the perspective of supply chain.