We put forward a new balanced optic bridge reference technique in an optic fiber gas sensor. In the system, a loss-compensated signal carryiny gas concentration is independent on the variation of optic source intensity, fiber losses and the response of detectors. Also, considering the practical situation such as splitter states, the temperature features of the system etc, we modify the detected concentration signal equations by introducing correction parameters to improve system properties. A software filter program is also introduced by a microcomputer far from the spot.