An ideal high-loading carbon-sulfur nanocomposite would enable high-energy-density lithium-sulfur batteries to show high electrochemical utilization, stability, and rate capability. Therefore, in this paper, we investigate the effects of the nanoporosity of various porous conductive carbon substrates (e.g., nonporous, microporous, micro/mesoporous, and macroporous carbons) on the electrochemical characteristics and cell performances of the resulting high-loading carbon-sulfur composite cathodes. The comparison analysis of this work demonstrates the importance of having high microporosity in the sulfur cathode substrate. The high-loading microporous carbon-sulfur cathode attains a high sulfur loading of 4 mg cm-2 and sulfur content of 80 wt% at a low electrolyte-to-sulfur ratio of 10 µL mg-1. The lithium-sulfur cell with the microporous carbon-sulfur cathode demonstrates excellent electrochemical performances, attaining a high discharge capacity approaching 1100 mA∙h g-1, a high-capacity retention of 75% after 100 cycles, and superior high-rate capability of C/20-C/3 with excellent reversibility.