The adjustability of differential mode gain for few-mode erbium-doped fiber amplifiers (FM-EDFAs) can be used for mode gain equalization and compensation for link mode-dependent loss, which can significantly improve the networking flexibility of mode division multiplexing systems. A few-mode optical fiber device integrating the isolator and the wavelength division multiplexer (FM-IWDM) is produced to build all-fiber FM-EDFAs. Two optical amplifying experiments using the codirectional and bidirectional pumps with a center wavelength of 1480 nm are carried out, and the maximum adjustable differential mode gain of 2.6 dB and 4.8 dB can be achieved by adjusting the pump power ratio under the pump total power of 200 mW.