A fiber gas sensing system using hollow-core photonic crystal fiber (HC-PCF) as gas cell is introduced.The gas cell is made by a HC-PCF with length of 40 cm and core diameter of 20 μm.The HC-PCF is connected with a single mode fiber (SMF) by a connector.There is a 50 μm gap on the connector,from which the environmental gas can diffuse into HC-PCF cell.Combining with wavelength tunable fiber ring laser technology,a compact and a real all-optical-fiber gas sensing system is proposed.Because of the characteristic of high power and narrow wavelength width of the probing light emitting from the fiber ring laser the system can detect gas with high sensing selectivity and thus the cross-effects usually encountered in the ordinary gas sensing method are reduced greatly.The properties of the system are demonstrated experimentally by detection of acetylene (C2H2).The experimental results demonstrate that the time for acetylene diffusing into the proposed HC-PCF gas chamber is proximately 6 min under natural free diffusion conditions.Approximate linear relation between the system responses and the acetylene volume fraction are experimentally demonstrated and the minimal detectable volume fraction of acetylene of 0.1% is also achieved by experiments.