Above all, they can avoid meddling in parliamentary politics, as the ECB did when Italy's silvio berlusconi government collapsed in 2011. Then they can lower their heads and hope for the best. The investment of securities provides an important means for the central bank to conduct financial macro-control, which is of great significance to the sustainable and efficient development of the national economy. The international monetary fund has strongly defended its gloomy forecast of the UK after brexit, saying a warning of a slowdown in growth is imminent. Under the theoretical framework of the theory, the theory of commodity market equilibrium and the monetary market equilibrium of Keynesian theory are unified. Marx's theory of interest rate decisions from the perspective of the source and essence of interest, taking into account the institutional factors in the role of interest rate decisions of interest theory, its theoretical core is the interest rate is determined by the average profit margin. Marx believed that under capitalism, interest is a part of profit and a form of conversion of surplus value. Historical costs differ from current values