We investigate an approach to measuring harmful gases by enhanced Fano resonance generated in a universal multimode waveguide-microcavity model. Dynamic Fano resonance is theoretically described and experimentally proved to be associated with the phase shift between two waveguide modes as well as their amplitude ratio and coupling coefficient. The spectra can be engineered to form a Lorentz dip, various Fano lineshape, and Lorentz peak by controlling the coupling point in both microbottle and surface nanoscale axial photonics platforms. In principle, the model can be applied to any class of whispering gallery mode microcavity device. With sharp asymmetric lineshape, our model can improve the sensitivity by 51.5 times in theory when measuring the harmful gas refractive index change, which may open up opportunities for advancements in the harmful gas leakage detection applications.