We show experimentally that a tandem-pumped Yb-doped fiber amplifier with high power and low quantum-defect can be reached. And a high power 21 W output power at 1018 nm is demonstrated finally. We use this 1018 nm fiber laser as a pumping source of the 1080 nm fiber laser to demonstrate the tandem-pumped double-cladding Yb-doped fiber amplifier, and obtain an output power of 18.6 W at 1080 nm with a slope efficiency of 90.86%.