To effectively prevent systemic financial risks, it is necessary to strengthen the supervision of systemically important financial institutions. The successive promulgation of systemically important bank supervision policies has made systemically important banks in China face higher regulatory requirements for core tier 1 capital. The index method is used to identify 18 systemically important banks in China in 2020 from 40 large commercial banks. Among them, except for the four major state-owned commercial banks, Bank of Communications and China Merchants Bank, most of the banks have a small distance between their actual core tier 1 capital adequacy ratio and regulatory requirements, and are facing greater pressure of capital supplement. Based on the data of 17 systemically important banks in China from 2010 to 2020, the relationship between the source of core tier 1 capital and bank profit efficiency is analyzed by heteroscedasticity random boundary model. The results show that supplementing core tier 1 capital through internal capital accumulation helps improve the profit efficiency of systemically important banks in China, and the level of profit efficiency increases with the increase of the proportion of internal capital accumulation. However, supplementing core tier 1 capital through external financing will reduce the profit efficiency of systemically important banks in China, and the level of profit efficiency will decrease with the increase of the proportion of external financing. Therefore, systemically important banks in China should take the strategy of accumulating internal capital first and then external financing to supplement core tier 1 capital to achieve a win-win situation of risk prevention and efficiency improvement. At present, the four major state-controlled commercial banks, Bank of Communications, and China Merchants Bank, which have a small core tier 1 capital gap and high-profit margins, should adopt a capital replenishment strategy that focuses on internal capital accumulation and supplemented by external financing, while other banks can adopt a capital replenishment strategy that focuses on external financing and supplemented by internal capital accumulation.