Extreme capital flows have played an important role in successive financial crises; in other words, extreme capital flows are often the source and manifestation of financial system risk spillovers. Based on the quarterly data of 48 countries and regions from 2009-2023, this paper analyses the relationship and endogenous mechanism between extreme capital flows and financial system risk spillovers from the theoretical and empirical levels, based on the index of financial system risk spillovers of each country and the interval of extreme capital flows (surges、stops、flights、retrenchments). The study finds that, first, capital stops and flights significantly exacerbate the level of risk spillovers in the financial system as a whole, while capital surges and retrenchments are conducive to mitigating risk contagion between financial sectors. Second, extreme capital flows have an asymmetric impact on the internal risk spillover effects of the financial systems of countries with different levels of economic development, different degrees of currency internationalization, and different exchange rate systems. Third, the implementation of macroprudential regulatory policies can significantly mitigate the impact of extreme capital flows, and for economies with a lower degree of capital account openness, the regulatory effect of macroprudential policy tools in conjunction with capital control measures is more significant. This study deepens the understanding of the impact mechanism of extreme cross-border capital flows on financial systemic risks, and provides important insights for China to improve its cross-border capital flow management system, curb external risk shocks, and promote a high level of financial openness.