Supply chain alliances between Chinese listed companies have proliferated in recent years. Based on this background, this paper explores the knowledge transfer between suppliers and customers (upstream and downstream firms) in supply chain alliances. This article takes 142 pairs of supply chain alliances formed by 229 Chinese listed companies from 2001 to 2021 as the research object. By using propensity score matching method and difference-in-differences regressions, we find that supply chain alliances can facilitate knowledge transfer between firms and accelerate mutual learning between suppliers and customers. Compared to firms without supply chain alliances, firms that enter into supply chain alliances increase the number of citations of their alliance partners' inventions, the number of inventions of their alliance partners and have more inventions that cited to alliance partners' knowledge. In addition, we classify each pair of supply chain alliance partners into supplier and customer and find that there is mutual learning rather than unidirectional knowledge transfer between suppliers and customers in supply chain alliances. The treatment effects from supply chain alliances are present in both suppliers and customers. Specifically, there is no significant difference between the supplier's citations that cite its customer partner's inventions and the customer's citations that cite its supplier partner's inventions. This result indicates that the effect of supply chain alliances in promoting knowledge transfer from suppliers to customers is as good as promoting knowledge transfer from customers to suppliers. Moreover, among a pair of supply chain alliance partners, the partner with lower-technological-diversity learns more knowledge of the partner with higher-technological-diversity. This paper provides theory and new empirical evidence for our insight into the knowledge transfer of supply chain alliances.