The disclosed sensor is gas sensor of optical crystal fiber with hollow core. The air sensor includes light source, light guiding gas-cell light path, reference light path of general optical fiber, and signal conversion part. Through general optical fiber, and light splitter, the light guiding gas-cell light path is connected to light source. Being setup at another end of the light guiding gas-cell light path and the reference light path, the said signal conversion part including photodiode, and phase-locked amplifier is in use for providing signal for circuit of monitoring external gas concentration. Optical crystal fiber with hollow core of containing gas permeable micropores in micro level is adopted in the light guiding gas-cell. Comparing current optical fiber gas sensor, the invention possesses advantages: simple system, easy of controlling absorbed light path, and coupling loss. The invention raises effective absorbed light path, and response seeped of parts so as to suit monitoring gas in site.