In recent years, supply chain risks and stability have become a focal point of public attention. However, there is no consensus on how exogenous shocks affect the sustainability of supply chain relationships, nor a clear mechanism of influence. This study uses data from all A-share listed companies in China from Q2 2018 to Q4 2021, constructing a “supplier–quarter–customer” relationship dataset, with the COVID-19 pandemic serving as an exogenous shock. The results show that after experiencing exogenous shocks, the sustainability of supply chain relationships actually strengthens. This suggests that companies may take measures to enhance supply chain stability and maintain existing relationships to ensure sustainability. Channel analysis reveal that trade credit serves as a channel for the impact of exogenous shocks on the sustainability of supply chain relationships, with companies adjusting trade credit supply to downstream customers to maintain and strengthen stability. Additionally, the impact of exogenous shocks on the sustainability of supply chain relationships varies with market concentration, product input heterogeneity, and firms’ ownership type. Therefore, companies should enhance supply chain relationship management, utilize trade credit as a risk buffer, and optimize the supply chain structure to reduce risk transmission and maintain sustainability.