The amplified spontaneous emission in the Yb band and thermal effect are the main obstacles to the power scaling of high-power Er/Yb-codoped fiber lasers. In order to obtain a high-power and high-efficiency fiber laser， a liquid cooling structure and the cascade co-pumping method are used to carry out experimental research on the erbium-ytterbium co-doped fiber laser. The results show that by using a 2.1 m large core diameter erbium-ytterbium co-doped fiber， a maximum output of 21.6 W at 1 548.9 nm is obtained with a pump power of 47.5 W， and the pump conversion efficiency is 45.7%. The slope efficiency before and after the Yb band oscillation is established are 48.4% and 56.2%， respectively.