This study examines the impact of corporate social responsibility (CSR) decoupling on investment efficiency through the lens of systems thinking, using 34,143 firm-year observations from Chinese listed firms between 2009 and 2022. CSR decoupling is conceptualized as a systemic misalignment between two interrelated governance subsystems: the externally facing legitimacy subsystem and the internally embedded strategic action subsystem. Drawing on legitimacy theory and systems thinking, we find that CSR decoupling significantly reduces investment efficiency, primarily through overinvestment, with no consistent evidence of underinvestment. Furthermore, this effect is amplified in tightly coupled supply chain systems and is especially pronounced in foreign-owned firms. The findings contribute to the integration of systems thinking into CSR and corporate governance research, emphasizing the role of structural coupling strength in shaping the consequences of symbolic–substantive misalignment. The study also offers managerial and policy implications for improving the alignment between external CSR communication and internal strategic execution to enhance investment discipline and long-term value creation.