Based on master oscillator power amplifier regime, a 976 nm laser diode pumped 20-m-long single-mode double-clad large-mode-area ytterbium-doped fiber amplifier at 1 120 nm was experimentally demonstrated.In the experiment, the injected power of the fiber seed laser was set to be 10 mW.When the diode laser pump power is increased to 1.5 W, the amplifier system begins to emit signal laser at 1 120 nm.The experimental results reveal that the signal output power increases slowly under low pump power level, however, when the pump power is beyond 3.4 W, the slope efficiency increases fleetly to 48.5%.Limited to the available launched pump power of 6.8 W, a maximum output power of 1.97 W is achieved.The total optical-to-optical conversion efficiency of the system is 29%.The measured center wavelength of the output signal laser is 1120.89 nm with a linewidth as narrow as 0.02 nm, which well preserves the characteristics of the seed laser.According to the parameters of the optical devices in the experiment, based on the steady-state-model of the double-cladding fiber amplifier and finite-difference-method, the relationship of the output signal power versus the diode laser pump power is theoretically calculated.The comparison between the experimental results and the theoretical results displays that the growth trend of the output signal laser power kisses well with the theoretical model.The system will be saturated when the pump power beyond 200 W, which reveals that the amplifier has a large space of power scaling up.