Based on the Vernier effect of the cascaded double ring resonator (CDRR) sensor, a sensor consisting of a microfluidic control system, a sensing ring, and a reference ring with a micro-heater for thermal tuning is proposed in this paper. In wavelength interrogation, a broadband spectrometer or a large tunable range laser is not required. The shift of the output spectrum caused by the refractive index change of the sample is converted into the change of the electric power by the thermal tuning heater. Due to the Vernier effect, the sensitivity of the sensor is 20 times higher than that of the single ring. The spectral envelope of the thermal-optic tuning of the sensor was fitted by a Gaussian function in the wavelength range of 8 nm. The experimental results showed that the sensitivity was 33.703 W/RIU, and the limit of detection was $1.34\times {10}^{-5}\mathrm{RIU}$.