Two dimensional MoS2 nanosheet is emerging as a promising nanofiller for the improvement of polymer properties, yet the low-cost, facile and green method should be further explored to satisfy the large-scale production of MoS2 nanosheets. In addition, MoS2 surface need to be functionalized to form good interfacial compatibility with polymer matrix. Herein, we developed a tannic acid (TA)-assisted aqueous exfoliation approach to simultaneously exfoliate and non-covalently functionalize MoS2 nanosheets, where TA not only facilitated the exfoliation process of MoS2 (the yield was 59.2%), but also stabilized MoS2 nanosheet dispersions for three months against reaggregation. Notably, benefiting from the strong hydrogen bonding interactions between TA-MoS2 nanosheets and PAN chains, the incorporation of 2 wt% TA-MoS2 nanosheets led to 70.4% and 54.2% improvements in tensile strength and breakage elongation for PAN fiber. Meanwhile, the introduction of 2 wt% TA-MoS2 nanosheets resulted in a 38.1%, 29.6% and 33.98% reduction in peak heat release rate, total heat release and peak smoke production rate of PAN composite fiber, respectively. Moreover, compared to pure PAN fiber, the toxic and flammable gases (such as HCN, CO and CH4) that generated from the flaming PAN composite fibers were significantly decreased, indicating the improved fire safety. Based on the gaseous and condensed analysis, the enhanced flame retardancy of 2 wt% TA-MoS2/PAN composite fibers were attributed to the superior radical quenching and intumescent charring capacities of TA as well as the catalytic charring and nano-barrier effect of MoS2. This work not only provides enormous possibilities for the green and large-scale production of MoS2 nanosheets but also further broadens their applications in high-performance nanocomposites.