Geopolitical changes bring about fluctuations in political relationships. This paper analyzes the impact of political relationship volatility on multinational corporations’ adoption of corporate social responsibility (CSR) as a strategic response from a dynamic perspective. Based on institutional theory and signaling theory, this study investigates the relationship between bilateral political relationship volatility and MNCs’ CSR behavior in the host country, as well as the moderating effects of bilateral political tension, MNCs’ internationalization experience, and digital distance. Using a sample of 120 MNCs from the Fortune Global 500 operating in China from 2012 to 2021, this paper employs a fixed-effects model for empirical analysis. The results show that the volatility of the political relationship between the home and host countries has a positive U-shaped impact on MNCs’ CSR behavior in the host country, with moderate volatility being most conducive to CSR activities. Moreover, increased bilateral political tension weakens this positive U-shaped relationship, turning it into an inverted U-shape. Similarly, longer operating tenure in the host country and greater digital distance between the home and host countries also flatten this positive U-shaped relationship. This paper extends the scope of CSR as a legitimacy signal, from the conventional contexts of customers and suppliers to the government under political relationship volatility. It enriches the theoretical research on MNCs’ non-market strategies in the context of geopolitical fluctuations and provides valuable guidance for MNCs to cope with geopolitical uncertainties.