S-doped mesoporous carbon fibres with an S content as high as 14.0 atom % (29.4wt. %) are synthesized by carbonization of sucrose using MgSO4-containing porous whiskers as templates and S source. The ultrahigh S concentration is obtained at an optimized condition, including a calcination temperature of 600°C, whisker templates with a smaller diameter and a relatively lower carbon source to template ratio (1:5). X-ray photoelectron spectroscopy demonstrates that the bonding configurations of S are mainly assigned to thiophene-S (C-S-C). Raman spectra demonstrate that the amount of defects rise with the increase of S content. As electrodes for supercapacitors, the specific capacitance of the S-doped mesoporous carbon fibres significantly increases as the S content increasing, reaching 221Fg−1 at the scan rate of 10mVs−1 with an area-normalized capacitance of 38μFcm−2. And the capacitance retention maintains 95% even after 500 cycles, exhibiting excellent cycling stability. Our results indicate that S-doped mesoporous carbons with ultrahigh S concentration can be potential candidates as high quality electrode materials for supercapacitors.