Abstract: Modal crosstalk and differential mode delay in mode-division multiplexing in few-mode fiber must be compensated for with adaptive equalization algorithm. For the purpose of decreasing complexity of adaptive equalization algorithm in long-haul few-mode fiber communication system, a multiple-input multiple-output equalizer with variable step-size frequency-domain-block least mean square(VS-FBLMS) algorithm was proposed to uncouple a 2×2 mode-division multiplexing system. Compromising the conflicting requirements of convergence speed and convergence performance, FBLMS adaptive algorithm was employed to update equalizer taps and modify step-size factor with variable step-size function. Furthermore, the algorithm decreased the computational complexity by using fast Fourier transform. In a 112Gbit/s high-speed communication simulation system over 1000km in few-mode fiber, signal Q² factor increased 3.7dB with same convergence speed. The performance of the algorithm was tested on field programmable gate array with a 100km few mode fiber communication system. The study demonstrates that the algorithm successfully uncouples the signals of mode-division multiplexing system and realizes fast convergence and low disorder.