Abstract: To address the limited detection range and high detection limit inherent in silicon-based microring resonator sensors, this study proposes a microring resonator structure assisted by a phase unit and gratings. A T-shaped phase unit is introduced into the bus waveguide to excite asymmetric Fano resonance, while gratings patterned on the sidewall of the ring waveguide suppress lateral modes. By tracking the Fano lineshape resonance under such mode suppression, sensing performance is substantially improved. Using the finite-difference time-domain method, the influences of grating period, etching depth, and duty cycle on side-mode suppression are systematically investigated. Through structural optimization, the free spectral range is extended to 39.19 nm, approximately three times that of a conventional microring resonator, while secondary side modes are effectively suppressed. In refractive index sensing evaluations performed in pure water and glycerol solutions of varying concentrations, the proposed structure exhibits pronounced resonant redshifts. Benefiting from the ultra-steep Fano lineshape resonance, an intensity sensitivity of 7.11×10³ dB/RIU and a detection limit as low as 1.41×10⁻⁷ RIU are achieved. Compared with conventional microring structures, this represents a nearly 3-fold improvement in sensitivity and a 70.5% reduction in the detection limit. The proposed phase-unit and grating-assisted microring resonator structure significantly lowers the detection limit while expanding the detection range, offering valuable insights for the design of highly integrated and high-precision silicon-based optical sensors.