Gas generation-based immunoassay is considered an attractive biosensing platform for the detection of biomarkers by incorporating the target recognition event with a catalyzed gas-generating reaction. Herein, an optical gas pressure sensor based on a silver/polydimethylsiloxane (Ag/PDMS) bilayer system was designed as a signal transducer to read the concentration of the detection target alpha-fetoprotein (AFP) quantitatively. In this proposed pressure-based assay, silicon dioxide (SiO2) nanospheres decorated with platinum (Pt) nanoparticles were coupled to detect antibodies by covalent linkage, and the captured antibodies were conjugated with magnetic beads via streptavidin-biotin interaction, simultaneously. When the AFP was loaded, the Pt-catalyzed hydrogen peroxide (H2O2) decomposition reaction was triggered to induce a significant increase in the reflectance signal of the Ag/PDMS bilayer gas pressure sensor (BGPS) due to a "wrinkled-specular" transition of the Ag film's surface. Under optimal conditions, the pressure-based biosensor exhibited a broad linear detection range from 0.05 to 132 ng mL-1 with a limit of detection (LOD) of 0.016 ng mL-1 for AFP, which satisfies the requirements for the clinical detection of AFP. Besides, the high specificity and detection accuracy of our Ag/PDMS BGPS also proved its feasibility for practical diagnosis.