Ultrahigh quality (Q) factor can be achieved by slightly breaking the symmetry of periodic structures, leading to the transition from symmetry-protected bound states in the continuum (BICs) to quasi-BICs (qBICs). However, it usually requires slight asymmetry because it follows inverse quadratic dependence on the degree of asymmetry. Herein, we numerically propose BIC resonances can be supported using an asymmetric grating consisting of two ridges with different widths in one unit cell. These BICs as a result of destructive interference between radiation channels are achieved by deliberately designing grating height or using oblique incidence in the structure with a large degree of asymmetry. In addition, by monitoring the wavelength shift of qBICs with changes in the surrounding medium, high sensing performance assisted by the BIC resonances is implemented. We achieve a sensitivity of 329 nm/RIU and an ultrahigh figure of merit (FOM) of 4.1×107 RIU−1 with a grating height of 798 nm for one of the qBICs. We also achieve a sensitivity of 479.6 nm/RIU and an ultrahigh FOM of 5.27×104 RIU−1 with an incident angle of 25.5 deg for other qBICs. We believe the structure and method for optical refractive index sensing are highly sought after in the fields of optical trapping, biological sensing, and miniaturized nanolasers.