The methods of fitting the envelope of the transmission in a cascaded double-ring resonator sensor were studied. Different fitting functions was used to fit the double-ring transmission spectrum envelope and the fitting error by the single ring and vernier effect was analyzed. The theoretic calculation shows that the detection error of the sensor obtained by Gaussian fitting of the envelope is only 0.41% when the detection limit is increased by 10 times. The experimental results show that for the wavelength detection method, the detection error is as high as 14.96% without fitted. However, the sensor detection error is only 0.45% with Gaussian fitting function. The simulation and experimental results prove that the Gaussian function is the best fitting method, which can further improve the detection limit and the detection accuracy of the sensor based on vernier effect.