Abstract: The impact of customer stability on the quality of corporate innovation was empirically examined using a sample of A-share listed companies in Shanghai and Shenzhen from 2007 to 2023, based on data from the top five customers. The mediation mechanisms of management expense ratio, financial expense ratio, and capacity utilization rate were investigated, and heterogeneity analysis was further conducted from the perspectives of corporate competitive capability and factor intensity. The results indicate that customer stability significantly enhances corporate innovation quality, with its mechanism primarily realized through two pathways: reducing innovation-related costs (cost reduction) and improving supply-demand matching efficiency (efficiency enhancement). In labor-intensive industries and among firms with weaker market competitiveness, the promoting effect of customer stability on innovation quality is more pronounced. Therefore, enterprises should elevate long-term customer relationships to a strategic level, leverage the knowledge and demand advantages of stable customer bases to optimize resource allocation and operational efficiency, and formulate differentiated customer relationship strategies based on firm type and industry characteristics. This will transform customer stability into a key driver for continuously improving innovation quality and core competitiveness. This study expands the theoretical perspectives on the antecedents of corporate innovation quality and the economic consequences of customer stability, providing theoretical support and large-sample empirical evidence for enterprises to achieve high-quality innovation through building stable customer relationships.