The optical absorption characteristic, which is one of the key parameters of two-dimensional black phosphorus when functioning as an optoelectronic semiconductor device, can be affected by the exciton binding energy. Field enhancement achieved by metal nanostructures can be used to adjust the photoelectric properties of low-dimensional materials. Here, we propose a new coupled field enhancement method by using metal nanorod arrays and Fabry-Perot cavity to improve the exciton-dominated optical absorption in few-layer black phosphorus. Our simulation results indicate that the exciton-dominated optical absorption can be improved to 84% (increased by 5.8-fold), while achieving a polarization-dependent optical absorption ratio of more than 30. Meanwhile, full width at half maximum of the field-enhanced formant and the polarization ratio can be improved by optimizing the geometric parameters of metal nanostructures. Our results reveal that coupled field enhancement of few-layer black phosphorus could provide a potential new scheme for tunable or polarization-sensitive optoelectronic devices.