This paper deals with the problem of active uncertainty attenuation control for the electric power stage in Fuel Cell Supercapacitor (FC-SC) hybrid electric vehicles subject to components uncertainties and load variations. A composite control strategy is adopted to estimate and compensate the mismatched lumped disturbances via a nonlinear disturbance observer. An active uncertainty attenuation controller is constructed by means of the backstepping recursive design method combined with Lyapunov stability theory to control the DC bus voltage. The rigorous stability analysis and numerical simulation results demonstrate that the DC bus voltage can track its desired value in a satisfactory precision and will perform a strong robustness against the involved parameters variations.