A novel multiscale reinforcement was prepared by the fast growth of carbon nanospheres (CNSs) onto the surface of carbon fiber (CF) under mildly hydrothermal reaction. The uniform layer of CNS with an average diameter of 85 nm produced on the fiber surface. Further, the structural analysis, surface morphology, and thermal decomposition behavior of CNS-CF reinforcement were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy combined with Fourier transform infrared spectroscopy and thermogravimetric analysis, respectively. Cement-based composites based on the multiscale CNS-CF reinforcement have been fabricated to evaluate their high-temperature resistance. CNS-CF/cement composites have a better resistance to the degradation resulted from exposure to elevated temperature up to 600 A degrees C than CF/cement composites and pristine hardened pastes, since their relative residual compressive strength is superior. The degrading mechanisms due to exposure to elevated temperatures were discussed and confirmed by using SEM and XRD. Results indicated that enhanced high-temperature resistance was attributed to the effective interlocking between CF and matrix due to (1) the presence of nanoscale CNS on the surface of CF and (2) the formation of microchannels in the matrix since CNS collapsed prior to CF after exposure to elevated temperatures.