Excellent optical absorbers are always characterized by high quality factors and perfect absorption; however, these absorbers usually encounter the ohmic losses due to traditional 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 work. Figure (a) shows the structural model of the designed Fabry-Perot cavity absorber, which consists of Ag as a substrate, a layer of the dielectric material Al ₂O ₃above the Ag substrate, and a high-refractive-index grating as the top dielectric layer Si ridge. By adjusting the thickness parameter dof Al ₂O ₃, the transformation of BIC into q-BIC is achieved. Specifically, 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%. In this work, 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. 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 proposed 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 of 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, the maximum modulation depth and the minimum insertion loss of the dual-frequency switch reach 99.4% and 0.0004 dB, respectively. These findings have significant implications in the fields of photonics, optical communication, and sensor technology.