A high-Q Mach–Zehnder interferometer (MZI)-coupled microring is presented for optical sensing with high sensitivity and a large measurement range. By optimizing the length difference between the two arms of the MZI coupler, the MZI-coupled microring with a high-Q factor and high extinction ratio is obtained. In the present example, the Q factor of the designed silicon-nanowire-based microring is as high as $1.8\times {10}^5$ when the silicon nanowire has a propagation loss $L=2\mathrm{dB}∕\mathrm{cm}$ . Due to this high-Q factor, the sensitivity for the change $\Delta n$ of the effective refractive index is about ${10}^{-5}–{10}^{-6}$ by measuring the shift of the resonant wavelength. Because of the wavelength dependence of the coupling ratio of the MZI coupler, it is possible to have only one resonant wavelength with a high extinction ratio in a very large wavelength span [i.e., the quasi-free spectral range of the MZI-coupler microring], which offers a very large measurement range covering the refractive index change of gas/liquid samples (e.g., $0⩽\Delta n<0.48$ in the present example).