White-fungus-like NiSx microspheres have been synthesized on a large scale by using a simple hydrothermal method. The influence of the reaction time and the surfactant on the final products was investigated, and the formation mechanism was discussed. The synthesized white-fungus-like NiSx microspheres were used firstly as fillers in the fabrication of NiSx/polyvinylidene fluoride (PVDF) composites. Relationships between the loadings of the NiSx and wave-absorption properties of the composites were analyzed. The loss mechanisms of NiSx/PVDF with different loadings were also discussed according to their dielectric and magnetic behaviors.