The structures of the intumescent charred layers formed from polyolefin (PO) blends with expandable graphite (EG) and/or the other free-halogen flame retardant (HFFR) and their flame-retardant mechanism were studied by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS), scanning electron microscopy (SEM), differential thermal analysis (DTA), and thermal conductivity (TC) measurements. The FTIR, XPS, and LRS data showed that the carbonaceous structures of intumescent charred layers consist of EG and various numbers of condensed benzene rings and/or phosphocarbonaceous complexes attached by the P-O-C and P-N bonds or quaternary nitrogen or dehydrated zinc berate (ZB). These results and the morphologic structures observed by SEM have demonstrated that the compact structures of charred layers slow down heat and mass transfer between the gas and condensed phase and prevent the underlying polymeric substrate from further attack by heat flux in a flame. The DTA data provide the positive evidence for the flame-retardant mechanism of the PO/EG/HFFR systems, which works by increasing the oxidation temperature and decreasing thermal oxidation heat. At the same time, the TC data reveal the flame-retardant essence of the charred layers as good heat-insulated materials whose TC value is only about 1/10 of the corresponding blend.