We provide an all fiber spectrally flat supercontinuum (SC) source generated by pumping a 20 m graded-index multimode fiber (MMF). The fiber pigtails of the all-fiber pump pulse laser and the MMF fused with a special method. After measuring the spectrum and the power, the output power extending from the visible to the mid-infrared is 2 W. To the best of our knowledge, this is the first watt-class and spectrally flat all fiber structured SC source based on graded-index MMF. Finally, we studied the evolution of the SC with increasing output power, which permitted us to more clearly recognize the nonlinear phenomena in the generation of the SC.