The microring resonator has received increasing attention in the optical sensing application because of its micro-size, optical property, and high sensitivity. An additional waveguide is commonly used to change the output spectra in the early research on microring resonators. In this study, we proposed a nested microring resonator that doubles the free spectral range (FSR) compared with the conventional single microring. This structure improved the sensing property as the FSR in the filter output spectra could be considered as a measurement range in the microring sensor. Moreover, the parameters including the coupling coefficient of the three coupling sections, length of the U-bend waveguide, and effective index of a waveguide were tested and carefully selected to optimize the sensing properties. The relationship between these parameters and the output spectra was demonstrated. With linear sensitivity, the structure has a good potential in sensing application.