Iron-based fluorides are promising alternates for advanced sodium-free battery cathodes due to their large theoretical capacity. However, the rational structural control on the iron-based fluorides toward high-performance batteries is still challenging. To this end, a controllable porous structure on FeF·0.33HO sub-microspheres is achieved by a polyethylene glycol (PEG)-assisted hydrothermal method via adjusting the volume of PEG-400. Experimental and molecular dynamic results verify that the formation of small amethyst-like sub-microspheres is mainly ascribed to the steric hindrance reaction of PEG-400, which makes it difficult for F to combine with Fe to form coordination bonds, and partially hinders the nucleation and growth of FeF·0.33HO nanospheres. As a sodium-free battery cathode, the FeF·0.33HO sub-microspheres with porous structure and smaller particle size exhibit excellent electrochemical performance with regard to cycle capacity and rate capability (a remaining capacity of 328 mAh g and up to 95.3% retention rate when backs to 0.1 C after 60 cycles).