In the optical gas sensor of the application, a three-dimensional reaction chamber structure is used to replace the traditional simple structure, so that the performance of the gas sensor can be enhanced in a wafer-level size. Besides, a light source, a reaction chamber and a light detector are integrated into one wafer in an exemplary embodiment, so as to achieve the wafer-level integration. In addition, the optical gas sensor can detect various gases simultaneously and has wide application in fields such as home environment monitoring, industrial safety, and disease diagnosis and treatment.