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The essay theoretically analyses the Fermi distribution of gradient refractive index waveguide cladding mode absorption spectrum characteristics, it also analyses the structure of the sensitivity within cladding absorption associated with the waveguide parameters, test sensitivity is associated with the waveguide mode and the waveguide parameters, especially with the waveguide layer thickness which is very sensitive to correlation. The analysis of results was confirmed by the Experiment of Guided Wave Loss Test. In the experiment the interaction ratio is measured, with waveguide in corresponding thickness and the mode in maximal interaction ratio is used as a sensing device which is used as a waveguide circuit with double-sample serial structure for measuring the liquid-clad absorption loss. The results show a very significant linear relationship between solution concentration and refractive index's imaginary part which characterizes the solution cladding absorption feature. By using this sensing relationship, the concentrations C. I. Pigment Green7 solution samples 13 with kinds of different concentrations have been measured. The standard deviation between the measured values and the actual values is less than 0.07%.