This research demonstrated the annealing-free synthesis of K-doped mixed-phase TiO2 (anatase and rutile, AR) nanofibers (K-TNF) on Ti foil at 150°C assisted by KOH(aq.) for electrochemical supercapacitors (ESCs) applications. The aggregated network and the average diameter of K-TNF was slightly decreased with the increase of KOH(aq.) concentrations from 4 to 6M, while the amount of K-doping, Ti3+ interstitials, and OH functional groups was substantially increased. The TiO2 phase was entirely rutile at 4M KOH(aq.), while it was a mixed AR phase at 5 and 6M. All the K-TNF modified Ti electrodes (K-TNF/Ti) exhibited quasi-rectangular shaped cyclic voltammograms (CVs) in a wide potential range and the specific capacitance (Cs) for the optimal electrode with mixed AR phase TiO2 was ca.102.12 and 97.30mF/cm2, obtained from the CV (scan rate, 5mV/s) and charge-discharge (CD, current density, 50μA/cm2) measurements, respectively. The higher Cs for the optimal K-TNF/Ti electrode can be ascribed to the synergistic effect of mixed AR phase, high percentage of K-doping (ca.17.20%), and Ti3+ interstitials (ca.16.20%), respectively. The directional electron transport through the 1D channel as well as the OH functional groups on the K-TNF surface is also contribute to enhance Cs. The K-TNF/Ti electrode is covered excellent stability with the Cs retention of ca. 91% and a very small change of internal series resistance (Rs) and charge transfer resistance (Rct) at the electrodeǀelectrolyte interface after 2500-CD cycles.