Based on the requirement and the influence of environmental factor on weapon system, two typical high energy fiber laser systems, coherent- and incoherent-combining types, were analyzed and compared. Beam propagation factor(BPF) was introduced as the beam quality factor for the combined beam. The influence of array element beam quality, atmospheric propagation, tracking jitter, phase control on the combined beam quality was numerically evaluated. The development trends of high energy fiber laser system were briefly discussed.