Tandem pumping is one of the main technical approaches to realize high power fiber lasers. By combining the improved chemical vapor deposition method with the solution doping technique, we have fabricated an Yb-doped double-cladding fiber. Through the design of fiber core components and the optimization of the rod making process, the uniformity of the refractive index of the highly doped fiber core is improved. The laser performance of the fiber is demonstrated by an all-fiber master oscillator power amplifying laser system. As for the 1081 nm fiber laser based on tandem pumping, a 20.88 kW laser output at 1080 nm with a high stimulated Raman scattering rejection ratio has been achieved with an 82.7% slope efficiency. This is the highest power achieved for the domestic fiber based on tandem pumping.