World is rich in unconventional oil and various alternatives to petroleum. However, conventional oil production declines so quickly that it is likely these unconventional oil resources cannot be put into production fast enough, and thus will not be compensated sufficiently. We realize detecting rapid detection of water content in heavy oil. The waveguide consists of a metal-insulator-metal (MIM) waveguide, rectangular cavity resonator, and crescent-shaped cavity resonator. The effects of the coupling distance, geometry of the crescent-shaped cavity resonator and its rotation angle, and length and width of the rectangular cavity resonator on the Fano resonance lines were numerically analyzed. Multiple Fano resonances can be produced as the rotation angle of the crescent-shaped cavity resonator is adjusted, and the sensor’s refractive index sensitivity was found to be \(935.71 \text{n}\text{m}/\text{R}\text{I}\text{U}\). By measuring the water content in heavy oil, we found that the Fano resonance lines shift toward shorter wavelengths as the volume fraction of water content increases. The detection resolution in heavy oil \(1.79\times {10}^{-9}\). The results presented here show that water content in heavy oil can be calculated using the measured change in the Fano resonance wavelength.