With the commercialization of the fifth-generation mobile communication technology, the global network traffic is exploding. However, due to the proximity to the Shannon limit, the capacity of the traditional single-core single-mode fiber communication system is unsatisfactory in the future network construction. In order to meet the upcoming challenges, it is urgent for us to upgrade the communication system for capacity expansion. To this end, fiber communication systems using space division multiplexing (SDM), dividing the channels in the spatial dimension, can accelerate the capacity expansion of communication systems. In an SDM system, the fiber amplifier has always been one of the most indispensable devices and has become a research hotspot. In the amplifier, when the power gain is unbalanced among channels, the power difference will accumulate with the transmission distance, leading to an increase in the outage probability and bit error rate at the receiver. Eventually, the transmission performance of the system is directly affected. Thus, achieving the gain equalization among different channels is a key to the SDM fiber amplifiers. In this paper, according to the theoretical and experimental researches, we review the gain equalization technology of SDM fiber amplifiers, including fiber structure design, refractive index doping profile optimization, and pump mode selection. Besides, the working principle, structural characteristics, and deficiencies are analyzed in detail, which provides solutions for the optimal design and performance improvement of SDM fiber communication systems.