In this paper, we present a novel gas sensing mechanism leveraging mode localization and adsorption expansion to achieve high-sensitivity detection of low-concentration gases. Unlike traditional resonant structures, our approach amplifies the frequency shift through the combined effects of changes in the sensing beam's mass and stiffness. Furthermore, by utilizing mode localization, the frequency shift is converted into modal amplitude variation, providing a second stage of sensitivity amplification. We derive the dynamics of the coupled resonator mode and perform MATLAB simulations to validate our approach. Theoretical results indicate that a higher absorption-expansion coefficient of the functional film leads to greater sensitivity amplification. These findings are corroborated by equivalent experimental results. Experimental analysis proves a striking sensitivity amplification, with the amplitude ratio sensitivity exhibiting a 20-fold increase over the relative frequency change sensitivity. Subsequent to the sensitivity enhancement in preliminary, our ongoing research endeavors are poised to focus on the design and production of tangible microelectromechanical system (MEMS) sensors, with an intensified emphasis on the refinement of absorption-expansion sensing technology. These sensors are anticipated to hold immense potential for diverse industrial applications necessitating precise and discerning detection capabilities.