Compared with the symmetric Lorentzian line shape, asymmetric resonant Fano line shape can realize dramatical variation of optical transmission intensity in the integrated optical circuits, which can effectively improve the sensitivity of optical switches, modulators, and sensors. A Fano resonator based on grating-assisted microring resonator is proposed. The resonator is made of silicon-on-insulator, and the Fano resonance is realized by adding two groups of waveguide grating structures in the runway microring. Based on the transfer matrix theory, the output spectra of the Fano resonator is deduced, the effects of variation of structural parameters in the resonator on the resonant wavelength, notch depth, and slope of the Fano spectra are analyzed. A Fano spectrum with the slope of -299.67 dB/nm, notch depth of 9 dB, and insertion loss of 6.4 dB is realized. The Fano resonator has advantages of high slope, small size, and simple manufacture, which can be widely used in the fields of optical switching, optical sensing, and optical detection.