A novel method involving the fabrication of Mo–W mixed oxide (Mo x W1–x O3) is proposed to modify the modest reaction kinetics and poor cycling stability of MoO3 material. By a simple coelectrodeposition method, a series of Mo x W1–x O3 oxides is deposited on a TiO2 nanotube array substrate. Because of the differences between Mo6+ and W6+ in nature, there is significant distortion existing in the mixed oxides, leading to their decreased crystallite size and enlarged lattice space, which facilitates ion diffusion in the solid. As results, the mixed oxides show much better balance between specific capacitance and cycling stability than the bare MoO3 or WO3 sample, which suffers from either poor cycling stability or low electrochemical activity. Impressively, the optimal Mo–W mixed oxide exhibits a high specific capacitance of 517.4 F g–1 at 1 A g–1, and, moreover, it retains 89.3% of the capacitance even at a high current density of 10 A g–1, demonstrating ultrahigh rate capability. These findings reveal the potential of the Mo–W mixed oxide for constructing advanced ultrahigh power supercapacitors.