The considerations in the epitaxial and longitudinal design of a supper-large-optical-cavity structure diode laser in the 976-nm band are numerically studied and presented here. Mode control layers were designed underneath and up the quantum well layer to suppress the lasing of high-order transverse modes. The electron leakage was suppressed by a band energy engineering， where the electron barrier increases from the p-waveguide layer to the p-cladding layer. The optimized structure has an internal loss of 0.66 cm^-1， an internal quantum efficiency of 0.954， and a full width at half maximum vertical far-field angle of 17.4°. For the resonant cavity design， a liner current profile along the cavity was proposed to reduce the longitudinal spatial hole burning effect， where a power penalty of 1.0 W at 20 A is suppressed. The 4-mm-long and 100 μm wide broad-area single emitter with the supper-large-optical-cavity epitaxial structure was designed to have a high power-efficiency of about 71% at an output power of 21 W under continuous current injection at 25°C.