A novel variable optical attenuator was developed and a detecting device was designed for testing its coupling characteristics.Based on coupling theory,the relations between the attenuation and the changing parameters were analyzed and the wavelength minim-loss was discussed.The experimental results indicate that the attenuation can change sharply and approach to 30 dB through adjusting the lateral displacement u of two tapered fibers,the attenuation can change slightly and reach 37 dB by adjusting the longitudinal coupling length L of two tapered fibers.And this variable optical attenuator can obtain the wavelength minim-loss less than 0.2 dB.By this new design structure,the light signal can be weakened or controlled ideally.The reliability and singleness can be guaranteed using its wavelength minim-loss characteristic when the gas concentration measurement and identity are carried out.