Electrocatalytic N2 reduction as an alternative approach to the energy-intensive and large CO2-producing Haber-Bosch process for NH3 synthesis under mild conditions has attracted extensive attention. Current research efforts on N2 reduction have mainly focused on metal-based catalysts, but metal-free alternatives can avoid the issue of metal ion release. In this work, oxygen-doped hollow carbon microtubes (O-KFCNTs) derived from natural kapok fibers are reported as a metal-free NRR electrocatalyst for N2-to-NH3 conversion with excellent selectivity. In 0.1 M HCl, the O-KFCNTs achieve a high faradaic efficiency of 9.1% at -0.80 V vs. a reversible hydrogen electrode (RHE) and a NH3 yield rate of 25.12 μg h-1 mgcat.-1 at -0.85 V vs. RHE under ambient conditions. Notably, this catalyst also demonstrates high stability.