An environment refractive index sensor composed of two identical coupling microfiber was demonstrated.Due to the presence of evanescent-wave at the surface of microfiber,the slight variation of the environment refractive index outside the microfiber will result in the change of the evanescent-wave and therefore results in the redistribution of energy in two arm of the coupler.In the experiment,the concentration of the glucose solution was sensed by measuring the ratio of output powers at two ends of the coupler.The experiment result shows that the sensitivity of this sensor is as high as 1.58 mol·L－1.The relationship between coupling efficiency and the environment refractive index is analyzed in detail based on a coupled mode theory.It is found that the sensitivity of this sensor can be further enhanced by decreasing the diameter of microfiber,or by increasing the wavelength of the incident light,or by increasing the length of coupling region.