Conjugated polymers of organic carbonyl compounds are promising electrode materials for energy storage devices owing to the renewable development prospects, structural variability, and better insolubility in electrolyte. However, the synthesis methods in solution are cumbersome and complicated in separation and purification, and require the introduction of functional groups and use of expensive catalysts. In this work, a novel conjugated poly(3,4,9,10-perylenetetracarboxylic diimide) (PPI) with superior thermal stability and lower solubility was prepared successfully by a green facile mechanical ball milling strategy. The PPI exhibits enhanced electrochemical dynamics performance, preferable rate capability, higher discharge capacity, and excellent cycling stability of 450 cycles at 0.2 C with higher capacity retention of 85.7% when used as cathode material for sodium-ion battery. Furthermore, the in-situ X-ray diffraction (XRD) and in-situ Raman investigations combined with the Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) were carried out to investigate the sodium storage mechanism. The results indicate that only two sodium ions are bound to two opposite carbonyl groups of PPI monomer to form sodium enolates during normal charging and discharging and to deliver available reversible capacity.