With the increasing emphasis on atmospheric environmental protection, it is crucial to find an efficient, direct, and accurate method to identify pollutant species in the atmosphere. To solve this problem, we designed and prepared the cascade multicavity (CMC) structure composed with silver nanoparticles (Ag NPs) as a surface-enhanced Raman scattering (SERS) substrate with favorable light transmittance and flexibility. The multicavity structure distributed on the surface introducing the homogeneous connecting holes endows the structure to more fully utilize the incident light while slowing the gas movement rate. Theoretical and experimental results have demonstrated that the Ag NPs/cascade multicavity (Ag-CMC) SERS substrate is a highly sensitive SERS substrate that can be used for in situ detection of gases under non-perpendicularly incident laser irradiation or bending of the substrate. We believe that the SERS substrate can provide a more efficient and feasible way for in situ detection of gaseous pollutants.