Excellent optical absorbers are always characterized by high quality factors and perfect absorption; however, these absorbers usually suffer from the ohmic losses due to conventional surface plasmon resonance, which limits their absorption performance in practical applications. To address the problem, a tunable bound state in the continuum (BIC) based on Fabry-Perot cavity is proposed in this paper. Figures 1(a) shows the structural model of the designed Fabry-Perot cavity absorber, which consists of Ag as the substrate, a layer of the dielectric material Al ₂O ₃above the Ag, and a high-refractive-index grating as the top dielectric layer Si ridge. By adjusting the thickness parameter dof Al ₂O ₃, the conversion of BIC and q-BIC is achieved in this paper, and when dis increased from 273 nm to 298 nm, the BIC can be transformed into quasi-BIC, and the perfect absorption of the absorber in the continuum spectrum can be increased to 100%, as shown in Figs. (b) and (c). In this paper, the factors affecting the perfect absorption are explored by using the interference theory; theoretical calculations of the quasi-BIC are carried out by using the coupled mode theory and impedance matching theory; the physical mechanism of the BIC is explained by using the electric and magnetic field theory, and the BIC is caused by the electric and magnetic dipole modes as well as the mirror image of the base Ag, which causes the interferential phase cancellation effect. Compared with the conventional absorber, the absorber has excellent structural parameter robustness and a wide range of BIC modulation. More importantly, the absorber has excellent sensing performance with a maximum sensitivity up to 34 nm/RIU and a maximum quality factor of 9.5. Last but not least, the absorber also achieves dual-frequency open-light performance, where the maximum modulation depth and the minimum insertion loss of the dual-frequency switch are 99.4% and 0.0004 dB, respectively. These findings have important implications in the fields of photonics, optical communications, and sensor technology.