In this letter, we theoretically investigate the physical mechanism of bilayer (BL) borophene synthesized on Ag (111) film, stimulating by recent experimental report [Borophene synthesis beyond the single-atomic-layer limit. Nature Materials, 21, 35–40 (2022)]. Our results reveal that strong hybrid coupling interaction and interfacial charge transfer between BL borophene and Ag (111) film results in the interlayer covalent bond by the van der Waals interaction. Interlayer covalent chemical bond results in the two Dirac points in BL borophene, as electric cage, which can efficiently confine electrons. We propose the potential applications of BL borophene for optical sensor and thermoelectric devices, based on the physical properties of BL borophene.