We report the fabrication of lightweight MWCNTs-g-PAN carbon fiber precursors as high absorptivity and wide-bandgap novel materials. The synthesis of carbon fiber precursors was performed via the grafting process of polyacrylonitrile on the surface of acid functionalized multiwalled carbon nanotubes. 1H NMR and FTIR spectroscopic techniques revealed a successful grafting process, and the formation of MWCNTs-g-PAN fibers with the extent of stabilization (i.e., degree of conjugation) equals 3.0. Such carbon fiber precursors with 200–600 μm thickness demonstrate excellent thermal stability with three degradation steps at 276, 413, and 850 °C, respectively. Carbon fiber precursors display selective high absorption UV radiation and sharp discrete energy difference at 284 nm. They demonstrate linear dependence of absorptivity (ε) on weight fraction of multiwalled carbon nanotubes in the carbon fiber precursors (χCNTs). Large absorptivity of UV radiation occurs using χCNTs (%) = 0.50 specimen due to long chain aligned nanofibril structures, whereas small absorptivity of UV radiation occurs using χCNTs (%) = 8.26 specimen due to wrap-up of short length PAN chains around carbon nanotubes forming flattened and thicker nanotubes, as depicted by ESEM images. Sensitive cutoff UVB absorptions correspond to 4.36–4.38 eV, which classify carbon fiber precursors as promising wide-bandgap materials and probably adapt them to play a focal role in many future optoelectronic and/or aeronautic implementations.