The invention relates to an instrument used for measuring the concentrations of liquids and gases and having distinguishable refractive index change of [delta]n-2.0*10<-7> magnitude. The instrument comprises an ultra-thick hollow core coated waveguide, a laser device, a CCD detector, a peristaltic pump for the entering, the exiting and the rinsing of a sample, and a data display device, and is characterized in that the thickness of the waveguide layer of the ultra-thick hollow core coated waveguide is about 5 mm, and the resonance angle of the minimum guide mode through the free space coupling is close to 00 so as to achieve the effective refractive index N(right arrow)0, such that the great measurement sensitivity is obtained. According to t he present invention, the coupling elements such as prisms and the like are not required, the guide mode is directly excited by using the free space coupling, the testable sample has the wide refractive index range and can be gas or liquid, the sample having the high refractive index is not restricted, the guide model is excited with the incidence angle of close to 00 to be adopted as the probe so as to provide the extremely high sensitivity, and by using the non-mobile way to obtain the signal, the slight refractive index change of the sample can be real-timely obtained while the influence caused by the external vibration is low.