Band gap engineering was achieved by in situ doping method for high electrical conductivity and chemical activity of MnO₂. By in situ releasing and adsorption during electrodeposition, Al³⁺ with close ion radius to Mn⁴⁺ could replace the position of Mn⁴⁺ in MnO₂. The in situ doping process brings impurity level in MnO₂ and changes the energy band structure. The narrower band gap of MnO₂ after Al doping with higher electron concentration on conduction band could improve the conductivity of MnO₂. The specific capacitance of Al-doped MnO₂ achieves 430.6 F g⁻¹ which is almost 2.5 times of the capacitance of initial MnO₂ (177 F g⁻¹), shedding light on its practical applications.