A portable optical-fiber-sensing apparatus for detecting concentration of flammable and explosive gases and hazardous gases relates to applications of optical fiber sensing to the field of security monitoring on flammable and explosive gases and hazardous gases of coal mines, non-coal mines and petrochemical depots. An optical sensor is used for detection of methane, hydrogen sulfide, carbon monoxide, carbon dioxide or alkanes oil gas, instead of a catalytic combustion sensor or an infrared sensor. The apparatus is high in precision, rapid in response, not affected by temperature and humidity, non-electric intrinsically safe, and long in service life. The optical fiber gas concentration sensing system consists of an optical fiber sensor, an optical-path circuit module and a system software. Different gases are modulated with different-wavelength lasers, and the lasers are transmitted to an open-type gas chamber via an optical fiber. Multiple pairs of lens reflectors are disposed in the gas chamber and make the lasers reciprocate in the gas chamber, and the lasers are interfered by a to-be detected gas and return to the optical-path circuit module for analysis, and are converted into electrical signals which are processed by the portable gas detection apparatus for uploading and alarming. The optical fiber sensor comprises an outer filter screen, the open-type gas chamber and the multiple pairs of lens reflectors.