We propose and simulate a device structure of resonant cavity-enhanced quantum-dot infrared photodetector (RCE-QDIP). The RCE-QDIP consists of a conventional n-i-n QDIP sandwiched by a bottom GaAs/Al2O3 distributed Bragg reflector and a top mirror of Ge/SiO2 sub-wavelength grating. Aiming for detecting mid-infrared at 8 μm, the total thickness of the device is only ∼7.7 μm. According to our simulation, the external quantum efficiencies of RCE-QDIP could be as high as 59%-78% with the enhancement factors of 7-30, compared with a conventional QDIP. The proposed RCE-QDIP is highly feasible as the various fabrication parameters are considered.