Waveguide structure model of fused taper fiber coupler in micron dimension was constructed. The optical power distribution change in fused taper fiber coupler model was gotten by numerical simulation with beam propagation method,and change rule of coupler output spectrum response was analyzed and obtained in different refractive indexes. According to fiber evanescent field coupling theory in micron dimension,the sensing experimental platform of fused taper fiber coupler was set up,and the changing relationship between transmission spectrum and surrounding sucrose solution refractive index was studied. The theoretical simulation and the experimental results show that with surrounding sucrose solution refractive index ranging from 1.3330～1.4000,transmission spectrum shift of sensing system has good linear relationship with surrounding refractive index changes,and the response sensitivity reaches 2272.7569 nm/RIU. This refractive sensor system has merits of wide measurement range,fast response,small size and simple fabrication.