A double-layer petal-structure optical antenna for mid-infrared detection is presented in this paper. The finite-difference time-domain method is used to analyze the effects of structural parameters and polarization direction of incident light on the resonant wavelength of the antenna and intensity of the electric field at the tip of the antenna. Based on the optimization of a single-layer structure, the ratios of the intensity of the electric field at the tip of the upper antenna to the intensity of the incident light are calculated with different incident wavelengths when the gap (h) between the two antennas is 0.1-0.8 μm. To investigate the enhancing mechanism of the lower antenna on the electric field of the upper antenna, the variations of the electric-field intensity ratio of the same measured point with and without the upper antenna are analyzed under fixed incident wavelength and an enlarged gap h (0.1-3.6 μm). The results indicate that the enhancement of the electric field at the tip of the upper antenna is mainly attributed to the coupling effect of the double-layer antenna structure, with h being less than 1 μm. When h is less than 0.2 μm, the electric-field intensity at the tip of the upper antenna reduces with decreasing h. This is because the energy at the tip of upper antenna is transferred to the interlayer region owing to near-field coupling. However, when h exceeds 1 μm, the enhancement of the electric field at the tip of the upper antenna is mainly ascribed to the interference effect of the reflected light from the lower antenna.