The attenuation detectable device of a Variable Optical Attenuator (VOA) is designed. The basic enactments of the two tapered fiber parameters are: The fiber core radius a =20

the lateral displacement u =158

the longitudinal superposed length L =100

the tapered angle

the fiber core refractive rate =1.47

and the envelope refractive rate =1.46. Then

based on coupling theory

through changing one parameter above

the relations between the attenuation and the corresponding parameter are analyzed

and the wavelength minim-loss characteristic which can guarantee the reliability and singleness of the gas concentration when the gas concentration measurement and identity are done is discussed.