A novel humidity sensor of polyimide(PI) based on the series coupled two-micro-ring resonators is proposed in the present paper. The transfer function of the micro ring resonator was calculated by using the transfer matrix method and the coupled mode theory. The authors compared the output spectrum characteristics of the traditional single micro-ring and series coupled two-micro-ring with different radii. The refractive index of the PI waveguide changes with different environmental humidity and this will lead to the drift of the output spectrum of the micro-ring resonator. By detecting the drift of the output spectrum we can measure the humidity, and the sensitivity and the sensing-range of the sensor are acquired accordingly. We also analyzed the output spectrum characteristics of resonators at different humidity sensing part. The theoretical results show the good performance of humidity sensor which could be used as the optimum sensing unit when the whole structure of the series coupled two-micro-ring resonators serves as the sensing part. The sensing-range and sensitivity of the system are improved by series micro-ring resonators of different radii compared to the conventional sensor with single micro-ring resonator. The free spectral range (FSR) of resonator reaches to 0.15 μm, the sensing-range is 10%RH～80%RH, and the sensitivity is 0.001 7 μm·(%RH)-1. Series coupled two-micro-ring with different radii gives theoretical instruction for producing integrated humidity sensor with low-cost, simple structure and high sensitivity.